# What's the mass of the interstellar neighborhood of the Sun?

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What's approximatively the mass of the interstellar neighborhood of the Sun ?
(for example the neighborhood of radius 50 light-years).

Remark: The mass of the observable matter should be completed by the mass of the "dark matter".

You don't have to guesstimate to come up with the answer.

What you do is look at the dynamics of stars with respect to the Galactic plane - in particular, the velocity dispersions of stars with known distances from the plane, combined with a reasonable assessment of where the Sun is with respect to the plane (close), yields an almost model-independent assessment of the mass density in the disk of our Galaxy in the vicinity of the Sun.

This work was done using the Hipparcos cataogue by Creze et al. (1998). They found that the local mass-density (all forms of matter) in the Galactic disk was $0.076 pm 0.015 M_{odot}/pc^3$.

Thus within 50 light years (15.3 pc), because the Galactic disk has an exponential scale height much bigger than this (about 100-200 pc), we can assume a constant density and derive a total mass in the solar neighborhood of $1100pm 200 M_{odot}$.

This mass is almost compatible with the total mass estimated to be in stars, white dwarfs, neutron stars, brown dwarfs and the interstellar medium gas. For instance, according to Chabrier (2001), the total mass density is almost accounted for in the form of 60% main sequence stars plus white dwarfs and neutrons stars, about 5% brown dwarfs and 30% gas, and so the contribution of dark matter to the local disk density is probably very small. Dark matter is not distributed in the same way as normal matter; it did not collapse to a disk, because it is dissipationless and probably exists in a spherical halo. So this consistency between the local baryonic matter density and the total mass density is not an argument against the dark matter hypothesis.

EDIT: I found a few estimates of the local dark matter density that use the kinematics of local stars as a constraint. The range of densities quoted is from 0.008 to 0.02 $M_{odot}$/pc$^3$ (Bovey & Tremaine 2012; Garbari et al. 2012) i.e. small compared with the contribution from stars and gas.

Very roughly: $3.5 imes 10^{33}kg$, or 1800 solar masses.

Here's how I came by that number, it is a very rough approximation.

The major mass components of the galaxy are stars, the interstellar medium, and dark matter.

According to the HYG Database there are approximately 1000 stars within 50 light years of the Earth. The average mass of a star is 0.2 solar masses (thanks to Rob Jeffries in the comments), where a solar mass is about $M_odot approx 2 imes 10^{30} kg$. This gives us $4 imes 10^{32} kg$ of stars nearby.

The interstellar medium (ISM) is primarily atomic hydrogen, and has an average density of 1 proton per cubic centimetre, although it can vary widely in different parts of the galaxy. A proton weighs $1.6 imes 10^{-27} kg$, so a 50 light year sphere of "average" ISM will weigh about $7 imes 10^{32}kg$. We can do a little bit better than that though. Our solar system lives in the local fluff which is a cloud in the local bubble. The local fluff has a radius of about 15 ly, and a density of 0.3 atoms per cubic centimetre. The local bubble is about 150 ly across, and has a density of only 0.05 atoms per cubic centimetre. Using these figures instead we get an approximate ISM mass of $4 imes 10^{31} kg$.

This is an order of magnitude smaller than the contribution of stars, and our estimate for the stars could easily be off by more than 10%, so let's err on the high side and say the total mass of stars + ISM is $5 imes 10^{32}kg$.

We don't know how much dark matter exactly is in the galaxy, but if its similar to the cosmological average then there is roughly six times as much dark matter as baryonic. If this holds true locally, then there is about $3 imes 10^{33}kg$ of dark matter nearby.

So, a rough estimate says there is about $3.5 imes 10^{33}kg$ of mass within 50 light years of us. This is equivalent to 1800 solar masses, or $2 imes 10^{60}$ protons!

## 20.6 Interstellar Matter around the Sun

We want to conclude our discussion of interstellar matter by asking how this material is organized in our immediate neighborhood. As we discussed above, orbiting X-ray observatories have shown that the Galaxy is full of bubbles of hot, X-ray-emitting gas. They also revealed a diffuse background of X-rays that appears to fill the entire sky from our perspective ([link]). While some of this emission comes from the interaction of the solar wind with the interstellar medium, a majority of it comes from beyond the solar system. The natural explanation for why there is X-ray-emitting gas all around us is that the Sun is itself inside one of the bubbles. We therefore call our “neighborhood” the Local Hot Bubble, or Local Bubble for short. The Local Bubble is much less dense—an average of approximately 0.01 atoms per cm 3 —than the average interstellar density of about 1 atom per cm 3 . This local gas has a temperature of about a million degrees, just like the gas in the other superbubbles that spread throughout our Galaxy, but because there is so little hot material, this high temperature does not affect the stars or planets in the area in any way.

What caused the Local Bubble to form? Scientists are not entirely sure, but the leading candidate is winds from stars and supernova explosions. In a nearby region in the direction of the constellations Scorpius and Centaurus, a lot of star formation took place about 15 million years ago. The most massive of these stars evolved very quickly until they produced strong winds, and some ended their lives by exploding. These processes filled the region around the Sun with hot gas, driving away cooler, denser gas. The rim of this expanding superbubble reached the Sun about 7.6 million years ago and now lies more than 200 light-years past the Sun in the general direction of the constellations of Orion, Perseus, and Auriga.

Figure 1. This image, made by the ROSAT satellite, shows the whole sky in X-rays as seen from Earth. Different colors indicate different X-ray energies: red is 0.25 kiloelectron volts, green is 0.75 kiloelectron volts, and blue is 1.5 kiloelectron volts. The image is oriented so the plane of the Galaxy runs across the middle of the image. The ubiquitous red color, which does not disappear completely even in the galactic plane, is evidence for a source of X-rays all around the Sun. (credit: modification of work by NASA)

A few clouds of interstellar matter do exist within the Local Bubble. The Sun itself seems to have entered a cloud about 10,000 years ago. This cloud is warm (with a temperature of about 7000 K) and has a density of 0.3 hydrogen atom per cm 3 —higher than most of the Local Bubble but still so tenuous that it is also referred to as Local Fluff ([link]). (Aren’t these astronomical names fun sometimes?)

While this is a pretty thin cloud, we estimate that it contributes 50 to 100 times more particles than the solar wind to the diffuse material between the planets in our solar system. These interstellar particles have been detected and their numbers counted by the spacecraft traveling between the planets. Perhaps someday, scientists will devise a way to collect them without destroying them and to return them to Earth, so that we can touch—or at least study in our laboratories—these messengers from distant stars.

Figure 2. The Sun and planets are currently moving through the Local Interstellar Cloud, which is also called the Local Fluff. Fluff is an appropriate description because the density of this cloud is only about 0.3 atom per cm3. In comparison, Earth’s atmosphere at the edge of space has around 1.2 × 1013 molecules per cm3. This image shows the patches of interstellar matter (mostly hydrogen gas) within about 20 light-years of the Sun. The temperature of the Local Interstellar Cloud is about 7000 K. The arrows point toward the directions that different parts of the cloud are moving. The names associated with each arrow indicate the constellations located on the sky toward which the parts of the cloud are headed. The solar system is thought to have entered the Local Interstellar Cloud, which is a small cloud located within a much larger superbubble that is expanding outward from the Scorpius-Centaurus region of the sky, at some point between 44,000 and 150,000 years ago and is expected to remain within it for another 10,000 to 20,000 years. (credit: modification of work by NASA/Goddard/Adler/University Chicago/Wesleyan)

## An Interstellar Ribbon of Clouds in the Sun’s Backyard

The clouds that make up the Radcliffe Wave (highlighted in red) pass within just 500 light years of our sun (yellow). Wave data has been superimposed on an artist’s rendering of the Milky Way galaxy as it appears in a screen shot taken from WorldWide Telescope.

Image courtesy of Alyssa Goodman, Harvard University

The clouds that make up the Radcliffe Wave (highlighted in red) pass within just 500 light years of our sun (yellow). Wave data has been superimposed on an artist’s rendering of the Milky Way galaxy as it appears in a screen shot taken from WorldWide Telescope.

Image courtesy of Alyssa Goodman, Harvard University

A 9,000-light-year-long ribbon of matter undulates through our sun’s interstellar neighborhood, made of hundreds of different clouds of dust and gas—the largest such structure of interacting nebulae yet described. Its discovery, announced today by a team of Harvard astronomers in the journal Nature, re-draws the map of our corner of the Milky Way and raises new questions about how stars and nebulae form and move through our galaxy, and those beyond.

The structure, dubbed the “Radcliffe Wave,” after Harvard’s Radcliffe Institute for Advanced Study, crests some 500 light years “above” the central disk of our spiral galaxy, before plunging just as far below. It holds about three million times the mass of the sun, mostly in clouds of dust and gas so diffuse they would register as a vacuum by any earthly standard. Never before have scientists seen interstellar clouds organized in a wave-like pattern like this one, but the team thinks that this wave is the backbone of the Orion Arm, the spiral arm of the Milky Way to which our sun belongs.

“We don’t know what causes this shape,” said João Alves, lead author of the Nature paper and a 2018-19 Radcliffe Fellow, in a statement. The professor of stellar astrophysics at the University of Vienna added, “[I]t could be like a ripple in a pond, as if something extraordinarily massive landed in our galaxy.”

“We have no precedent for this sort of structure in the galaxy,” said coauthor Catherine Zucker, a fifth-year doctoral student in astronomy at Harvard, in a separate interview, nor have wave-like structures such as this “been seen yet in simulations of galaxies like our Milky Way.”

The Harvard team found the wave after building the most accurate map to date of the interstellar clouds within about 7,000 light years of the sun. The map made it possible for the first time to visualize more than two billion cubic light years in three dimensions—and brought the massive wave into clear view. “I’m sure you’re wondering why, if this thing is right up in our face, we didn’t find it sooner,” coauthor Alyssa Goodman, Wilson professor of applied astronomy, said in a press conference. “It’s not apparent in 2-D” images of the sky. (Readers can explore an interactive view of the wave for themselves through 3-D visualizations that the authors have posted online.)

The authors chose the name ‘Radcliffe’ for the wave, they write, “in honour of both the early 20th century female astronomers from Radcliffe College and the interdisciplinary spirit of the current Radcliffe Institute that contributed to this discovery.” Alves is particularly indebted, he said, to Henrietta Leavitt, an 1892 Radcliffe graduate who worked as a “computer” at the Harvard College Observatory (see “Eye on the Cosmos,” November-December 2017, page 104). In 1908, Leavitt painstakingly compared the blinking of thousands of pulsing stars known as “Cepheid Variables,” and discovered a precise mathematical relationship between their brightness and the frequency of their pulses. Because knowing the true brightness of a star allows astronomers to calculate its distance based on its apparent brightness, astronomers would later use this discovery to produce the first precise measurements of intergalactic distances and the expansion of the universe.

Alves said he was inspired at the beginning of his Radcliffe fellowship by an exhibit showcasing the work of artist Anna von Mertens, including quilts sewn to show the stars at the moments of Leavitt’s birth and death (see “Looking at the Cosmos Through a Feminine Lens”). Of Leavitt herself, he declared: “Her incredibly detailed work opened the Universe for us.”

### Not a Ring, but a Wave

For more than a century, astronomers have grouped the interstellar clouds near the sun into a ring-like structure called Gould’s Belt, about 3,000 light years across.

The structure is named for Benjamin Gould, A.B. 1844, who in 1879 described a band of stars and nebulae forming an incomplete belt around Earth’s night sky. The belt stretches from the South Pole through constellations including Orion, near the equator, and Perseus, in the northern sky, and back south through Scorpio. Astronomers have long struggled to explain the gaps in the belt, and why it seemed to tilt relative to the disk of the Milky Way.

“One of the big issues with the Gould’s Belt model was that there was no consensus for how the structure actually formed,” said Zucker. Explanations ranged from a clump of dark matter plowing through the region to an ancient supernova in the center triggering star formation around the edges.

Alves, Zucker, and the rest of the team hoped a better map of Gould’s Belt and its surroundings would help answer these questions. “I was expecting Gould’s Belt in high definition,” said Alves. “I was not expecting any wave.” But when the team plotted out the data, it was clear: the most cohesive structure in our neighborhood is a line of clouds, remarkably straight from above but darting in and out of the Milky Way’s disk when viewed from the side.

The Radcliffe Wave provides an explanation for both the gaps and the tilt in Gould’s Belt: rather than a coherent structure, the “belt” is actually a straight, sloping portion of the wave (stretching through the northern sky, roughly from Orion to Perseus) and a clump of matter, unassociated with the wave, in the southern sky, between the constellations Scorpio and Centaurus.

“The Sun lies only 500 light years from the Wave,” said Alves in the statement. “It’s been right in front of our eyes all the time, but we couldn’t see it until now.”

The team deduced that the sun passed through the wave about 13 million years ago, and will do so again in another 13 million years. “There is no obvious record to my knowledge, no obvious mass extinction,” resulting from that event, said Alves. “So the impact might have been subtle,” perhaps merely a mist of iron ions and other supernovae residue settling on Earth, “but I suspect the event left a mark.”

Whether the sun’s passage through the wave left physical evidence or not, the sky at the time would have looked “fabulous,” Goodman said at the press conference. “All these beautiful nebulae and all this star formation would be all around us.”

### Interstellar Cloud Atlas

The map the team used to find the wave was based on data from the European Space Agency’s Gaia satellite, which has spent the last five years mapping the stars from its vantage point just beyond Earth’s orbit. Anyone can measure the direction to a star, but Gaia is measuring their distances as precisely as possible by triangulating them from different sites in its orbit.

Unlike stars, however, interstellar clouds like those in the Radcliffe Wave are too diffuse and dim for astronomers to measure their distance from Earth through triangulation. The research group of professor of astronomy and of physics Douglas Finkbeiner, of which Zucker is a part, got around this by using known star distances to measure cloud distances.

“There are stars that are in front of the cloud, and there are stars that are behind the cloud,” explained Zucker during the interview, “and we know the distances to both.” Because the clouds absorb more blue light than red light as starlight passes through them, stars behind the clouds appear dimmer and redder than they otherwise would. To determine the clouds’ distance, she said, “we can essentially bracket clouds between these background and foreground star formations.”

So far, the team has checked one clump of stars, formed from the nebulae in one part of the wave. “It is doing what would be expected,” said Alves: the stars furthest from the wave’s centerline are moving more slowly relative to the wave’s centerline than those closer in. But it’s not proof.

If the wave is indeed rippling through the galaxy’s disk, Zucker said, there are a few things that could have caused that disturbance, including the dwarf galaxies that circle the Milky Way and fast-moving clouds of gas that stream through it. “We think it’s possible that a high-velocity cloud or one of these dwarf galaxies hit the disk of the Milky Way, and that is what is causing this structure to undulate.”

To learn more, she continued, the team may try to run simulations “where we actually try to hit the Milky Way with objects of different mass from different angles and at different speeds and see if we can reproduce this structure.”

Understanding the wave, Zucker pointed out, will yield a better understanding of how stars form from gas clouds, like those within it. “People have been studying these clouds for hundreds of years, back before Gould. To find that these clouds are connected at a galactic scale transforms our understanding” of stars’ creation.

## Probing deep space with Interstellar

Scientists hope the proposed Interstellar Probe will teach us more about our home in the galaxy as well as how other stars in the galaxy interact with their interstellar neighbourhoods. Credit: Johns Hopkins APL

When the four-decades-old Voyager 1 and Voyager 2 spacecraft entered interstellar space in 2012 and 2018, respectively, scientists celebrated. These plucky spacecraft had already traveled 120 times the distance from the Earth to the sun to reach the boundary of the heliosphere, the bubble encompassing our solar system that's affected by the solar wind. The Voyagers discovered the edge of the bubble but left scientists with many questions about how our Sun interacts with the local interstellar medium. The twin Voyagers' instruments provide limited data, leaving critical gaps in our understanding of this region.

NASA and its partners are now planning for the next spacecraft, currently called the Interstellar Probe, to travel much deeper into interstellar space, 1,000 astronomical units (AU) from the sun, with the hope of learning more about how our home heliosphere formed and how it evolves.

"The Interstellar Probe will go to the unknown local interstellar space, where humanity has never reached before," says Elena Provornikova, the Interstellar Probe heliophysics lead from the Johns Hopkins Applied Physics Lab (APL) in Maryland. "For the first time, we will take a picture of our vast heliosphere from the outside to see what our solar system home looks like."

Provornikova and her colleagues will discuss the heliophysics science opportunities for the mission at the European Geosciences Union (EGU) General Assembly 2021.

The APL-led team, which involves some 500 scientists, engineers, and enthusiasts—both formal and informal—from around the world, has been studying what types of investigations the mission should plan for. "There are truly outstanding science opportunities that span heliophysics, planetary science, and astrophysics," Provornikova says.

Scientists plan for the Interstellar Probe to reach 1,000 AU -- 1 AU is the distance from the sun to Earth -- into the interstellar medium. That's about 10 times as far as the Voyager spacecraft have gone. Credit: Johns Hopkins APL

Some mysteries the team hopes to solve with the mission include: how the sun's plasma interacts with interstellar gas to create our heliosphere what lies beyond our heliosphere and what our heliosphere even looks like. The mission plans to take "images" of our heliosphere using energetic neutral atoms, and perhaps even "observe extragalactic background light from the early times of our galaxy formation—something that can't be seen from Earth," Provornikova says. Scientists also hope to learn more about how our sun interacts with the local galaxy, which might then offer clues as to how other stars in the galaxy interact with their interstellar neighborhoods, she says.

The heliosphere is also important because it shields our solar system from high-energy galactic cosmic rays. The sun is traveling around in our galaxy, going through different regions in interstellar space, Provornikova says. The sun is currently in what is called the Local Interstellar Cloud, but recent research suggests the sun may be moving toward the edge of the cloud, after which it would enter the next region of interstellar space—which we know nothing about. Such a change may make our heliosphere grow bigger or smaller or change the amount of galactic cosmic rays that get in and contribute to the background radiation level at Earth, she says.

This is the final year of a four-year "pragmatic concept study," in which the team has been investigating what science could be accomplished with this mission. At the end of the year, the team will deliver a report to NASA that outlines potential science, example instrument payloads, and example spacecraft and trajectory designs for the mission. "Our approach is to lay out the menu of what can be done in such a space mission," Provornikova says.

The mission could launch in the early 2030s and would take about 15 years to reach the heliosphere boundary—a pace that's quick compared to the Voyagers, which took 35 years to get there. The current mission design is planned to last 50 years or more.

Provornikova will present the latest on the Interstellar Probe heliophysics plan on Monday, 26 April at 14:00 CEST.

## Interstellar Matter around the Sun

We want to conclude our discussion of interstellar matter by asking how this material is organized in our immediate neighborhood. As we discussed above, orbiting X-ray observatories have shown that the Galaxy is full of bubbles of hot, X-ray-emitting gas. They also revealed a diffuse background of X-rays that appears to fill the entire sky from our perspective (Figure 1). While some of this emission comes from the interaction of the solar wind with the interstellar medium, a majority of it comes from beyond the solar system. The natural explanation for why there is X-ray-emitting gas all around us is that the Sun is itself inside one of the bubbles. We therefore call our “neighborhood” the Local Hot Bubble, or Local Bubble for short. The Local Bubble is much less dense—an average of approximately 0.01 atoms per cm 3 —than the average interstellar density of about 1 atom per cm 3 . This local gas has a temperature of about a million degrees, just like the gas in the other superbubbles that spread throughout our Galaxy, but because there is so little hot material, this high temperature does not affect the stars or planets in the area in any way.

What caused the Local Bubble to form? Scientists are not entirely sure, but the leading candidate is winds from stars and supernova explosions. In a nearby region in the direction of the constellations Scorpius and Centaurus, a lot of star formation took place about 15 million years ago. The most massive of these stars evolved very quickly until they produced strong winds, and some ended their lives by exploding. These processes filled the region around the Sun with hot gas, driving away cooler, denser gas. The rim of this expanding superbubble reached the Sun about 7.6 million years ago and now lies more than 200 light-years past the Sun in the general direction of the constellations of Orion, Perseus, and Auriga.

Figure 1. Sky in X-Rays: This image, made by the ROSAT satellite, shows the whole sky in X-rays as seen from Earth. Different colors indicate different X-ray energies: red is 0.25 kiloelectron volts, green is 0.75 kiloelectron volts, and blue is 1.5 kiloelectron volts. The image is oriented so the plane of the Galaxy runs across the middle of the image. The ubiquitous red color, which does not disappear completely even in the galactic plane, is evidence for a source of X-rays all around the Sun. (credit: modification of work by NASA)

A few clouds of interstellar matter do exist within the Local Bubble. The Sun itself seems to have entered a cloud about 10,000 years ago. This cloud is warm (with a temperature of about 7000 K) and has a density of 0.3 hydrogen atom per cm 3 —higher than most of the Local Bubble but still so tenuous that it is also referred to as Local Fluff (Figure 2). (Aren’t these astronomical names fun sometimes?)

While this is a pretty thin cloud, we estimate that it contributes 50 to 100 times more particles than the solar wind to the diffuse material between the planets in our solar system. These interstellar particles have been detected and their numbers counted by the spacecraft traveling between the planets. Perhaps someday, scientists will devise a way to collect them without destroying them and to return them to Earth, so that we can touch—or at least study in our laboratories—these messengers from distant stars.

Figure 2. Local Fluff: The Sun and planets are currently moving through the Local Interstellar Cloud, which is also called the Local Fluff. Fluff is an appropriate description because the density of this cloud is only about 0.3 atom per cm3. In comparison, Earth’s atmosphere at the edge of space has around 1.2 × 1013 molecules per cm3. This image shows the patches of interstellar matter (mostly hydrogen gas) within about 20 light-years of the Sun. The temperature of the Local Interstellar Cloud is about 7,000 K. The arrows point toward the directions that different parts of the cloud are moving. The names associated with each arrow indicate the constellations located on the sky toward which the parts of the cloud are headed. The solar system is thought to have entered the Local Interstellar Cloud, which is a small cloud located within a much larger superbubble that is expanding outward from the Scorpius-Centaurus region of the sky, at some point between 44,000 and 150,000 years ago and is expected to remain within it for another 10,000 to 20,000 years. (credit: modification of work by NASA/Goddard/Adler/University Chicago/Wesleyan)

### Key Concepts and Summary

The Sun is located at the edge of a low-density cloud called the Local Fluff. The Sun and this cloud are located within the Local Bubble, a region extending to at least 300 light-years from the Sun, within which the density of interstellar material is extremely low. Astronomers think this bubble was blown by some nearby stars that experienced a strong wind and some supernova explosions.

## The Faintest Dwarf Galaxies

Joshua D. Simon
Vol. 57, 2019

### Abstract

The lowest luminosity ( L) Milky Way satellite galaxies represent the extreme lower limit of the galaxy luminosity function. These ultra-faint dwarfs are the oldest, most dark matter–dominated, most metal-poor, and least chemically evolved stellar systems . Read More

### Supplemental Materials

Figure 1: Census of Milky Way satellite galaxies as a function of time. The objects shown here include all spectroscopically confirmed dwarf galaxies as well as those suspected to be dwarfs based on l.

Figure 2: Distribution of Milky Way satellites in absolute magnitude () and half-light radius. Confirmed dwarf galaxies are displayed as dark blue filled circles, and objects suspected to be dwarf gal.

Figure 3: Line-of-sight velocity dispersions of ultra-faint Milky Way satellites as a function of absolute magnitude. Measurements and uncertainties are shown as blue points with error bars, and 90% c.

Figure 4: (a) Dynamical masses of ultra-faint Milky Way satellites as a function of luminosity. (b) Mass-to-light ratios within the half-light radius for ultra-faint Milky Way satellites as a function.

Figure 5: Mean stellar metallicities of Milky Way satellites as a function of absolute magnitude. Confirmed dwarf galaxies are displayed as dark blue filled circles, and objects suspected to be dwarf .

Figure 6: Metallicity distribution function of stars in ultra-faint dwarfs. References for the metallicities shown here are listed in Supplemental Table 1. We note that these data are quite heterogene.

Figure 7: Chemical abundance patterns of stars in UFDs. Shown here are (a) [C/Fe], (b) [Mg/Fe], and (c) [Ba/Fe] ratios as functions of metallicity, respectively. UFD stars are plotted as colored diamo.

Figure 8: Detectability of faint stellar systems as functions of distance, absolute magnitude, and survey depth. The red curve shows the brightness of the 20th brightest star in an object as a functi.

Figure 9: (a) Color–magnitude diagram of Segue 1 (photometry from Muñoz et al. 2018). The shaded blue and pink magnitude regions indicate the approximate depth that can be reached with existing medium.

## The Diffuse Interstellar Bands

The diffuse interstellar bands are absorption features observed in the spectra of stars seen through significant column densities of interstellar material. Of the 127 confirmed DIBs in the optical region between 0.4 and about 1.3 /Lm, only 2 have (tentatively) been identified with a specific carrier. Because DIB strengths increase roughly in proportion to color excess, they were originally suspected of being produced on or in the interstellar grains, but current evidence favors some species of free polyatomic molecules, either neutral or ionized. DIBs are conspicuously broader than the atomic interstellar lines, having widths at half-depth ranging from 0.8 to about 30 Å. These widths are presumably due to unresolved rotational structure, possibly compounded by lifetime broadening of the upper state. Recent proposals that C60, some members of the PAH family, or polycarbon chains are responsible for the DIB spectrum are either not supported by observation or await better data.

## Interstellar medium

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Interstellar medium, region between the stars that contains vast, diffuse clouds of gases and minute solid particles. Such tenuous matter in the interstellar medium of the Milky Way system, in which the Earth is located, accounts for about 5 percent of the Galaxy’s total mass.

The interstellar medium is filled primarily with hydrogen gas. A relatively significant amount of helium has also been detected, along with smaller percentages of such substances as calcium, sodium, water, ammonia, and formaldehyde. Sizable quantities of dust particles of uncertain composition are present as well. In addition, primary cosmic rays travel through interstellar space, and magnetic fields thread their way across much of the region.

In most cases, interstellar matter occurs in cloudlike concentrations, which sometimes condense enough to form stars. These stars, in turn, continually lose mass, in some instances through small eruptions and in others in catastrophic explosions known as supernovae. The mass is thus fed back to the interstellar medium, where it mixes with matter that has not yet formed stars. This circulation of interstellar matter through stars determines to a large degree the amount of heavier elements in the cosmic clouds. Interstellar matter in the Milky Way Galaxy is found primarily in the system’s outer parts (i.e., the so-called spiral arms), which also contain a large number of young stars and nebulae. This matter is closely concentrated in a plane, a flat region commonly known as the galactic disk.

The interstellar medium is studied by several methods. Until the mid-20th century, virtually all information was obtained by analyzing the effects of interstellar matter on the light from distant stars with the aid of optical telescopes. Since the early 1950s, much research has been conducted with radio telescopes, which enable astronomers to study and interpret radio waves emitted by various constituents of the interstellar medium. For example, neutral (i.e., non-ionized) hydrogen atoms absorb or emit very small amounts of radio energy of a particular wavelength—namely, 21 cm. By being measured at this point and compared with nearby wavelengths, absorbing or radiating hydrogen clouds can be detected.

Optical and radio emissions have provided much of the information on the interstellar medium. In recent years, the use of infrared telescopes on orbiting satellite observatories has also contributed to knowledge of its properties, particularly the relative abundances of the constituent elements.

## Astronomy Beginner’s Guide Part 4: The Sun

The classical idea was that the Sun was a ball of white-hot iron, and in the 18 th century Sir William Herschel believed there was a dark, solid world, below the fiery clouds of the photosphere, which we saw through the vortices of sunspots.

The inhabitants carried sunshades to protect them from the heat overhead (at 6,000 o , they’d need them).

“Dr. Elliott in 1787 upheld this view, and on his trial at the Old Bailey for the murder of Miss Boydell, his friends maintained his insanity, and quoted as proof of their assertion the pages of his book, in which this opinion was expressed.” (J.E. Gore, “The Worlds of Space”, Innes, 1894.)

Gore was amazed that it had been revived “in modern times”, but in 2006 I was blamed for my ‘closed mind’, when I rejected it on the obvious ground that the Sun is mostly hydrogen and helium and its gravitational pull would be much higher otherwise.

Until the 1940s it was still believed that iron atoms outnumbered hydrogen ones in the solar core, but that has long since been disproved. (Simon Mitton, “Fred Hoyle, a Life in Science”, Aurum, 2005.) ESA satellites have found enhanced levels of iron in the solar atmosphere above sunspots, but that’s a far cry from saying there’s a solid layer below. [Michael Mozina, ‘The Surface (Ferrite Layer) of the Sun’, e-mail text, 2006.]

After analysis of their light, stars are grouped by spectral type. The types were labelled before the processes of nuclear fusion were understood now rearranged by surface temperature, the sequence from blue to red runs O, B, A, F, G, K, M, R, N, S (‘Oh be a fine girl, kiss me right now sweetie’).

The Hertzsprung-Russell diagram (above) plots stars by their light output (absolute magnitude) and surface temperature. The hottest, brightest stars (class O and B), are found at top left the faint red dwarfs (class M and below) at bottom right. Between them runs a diagonal band called the Main Sequence, on which all stable stars fall: our Sun is near the centre of the graph, classed G0 or G2. Stars are stable while fusing hydrogen to helium in their cores when the hydrogen is exhausted they move off the Main Sequence to upper right, becoming orange and red giants, increasingly unstable. The more massive stars go on to more violent fusion reactions some explode as supernovae, synthesising the heavy elements which form planets like ours the most massive collapse into black holes.

Storms on the Sun emit intense particle beams, associated with but not actually caused by the phenomenon of solar flares. Observations from the SOHO probe have shown that they aren’t shaped like jets from a hose, as we used to think. We now call them ‘Coronal Mass Ejections’, but they aren’t solid matter. They are plasma containing some nuclei of the heavier elements but consisting mostly of protons, with trapped electrons spiralling around the outside of the ‘jet’. Flares are huge releases of energy caused by the breaking of magnetic field lines and usually occur above sunspot groups, along with ‘coronal mass ejections’ which can have disruptive effects on Earth if they strike the magnetic field in the right position. For lunar astronauts they pose a major hazard when the Moon is outside the Earth’s magnetosphere, for about two-thirds of each month.

In the 1930s Sir James Jeans believed that the Solar System formed from such a streamer pulled out by a passing star, so rare an event that our planets would be unique. But nearly all the angular momentum in the Solar System is concentrated in the planets, where by Jeans’s mechanism it would still reside in the Sun. And now that we know of thousands of planetary systems, it’s clear that they tend to form along with stars and at the same time.

There’s a constant outflow of particles from the Sun called the Solar Wind, discovered by the Mariner 2 probe to Venus in 1962. Although very tenuous, that outflow from the ‘coronal holes’ in the Sun’s outer atmosphere has a major effect on the Earth’s magnetic field. Until Pioneer 10 passed Jupiter in 1974, it was thought that the boundary would be just beyond Jupiter’s orbit but Voyagers 1 and 2 only reached the outer edge of it 24 years and more after launch, and have only now emerged into interstellar space.

Aurorae and magnetic storms on Earth are caused by particle streams from storms on the Sun. The older model was that when the streams hit the Earth’s magnetic field, they were diverted into the Van Allen Belts of trapped radiation, overloading them. It now seems that the aurorae are caused by backlash waves in the Earth’s magnetic ‘tail’, entering the upper atmosphere along the ‘rings of fire’ surrounding the magnetic poles forming to form rippling structures of arcs, sheets and curtains of ionised gas – usually white, but higher energies generate the green light of ionised oxygen. Still more intense radiation generates a red glow higher in the sky, again due to oxygen, and still deeper penetration ionises nitrogen, which glows red in the lower parts of auroral arcs.

The dying geomagnetic storms interrupt communications and disrupt power supplies the worst recent one caused an 18-hour power cut in Quebec, in 1989. Recent evidence suggests that volcanic activity can also be affected. The mechanism for that isn’t obvious, although major episodes of continent-building in the distant past were correlated with resonances in the Earth’s outer core caused by the interacting gravitational pulls of the Sun and Moon.

Both flares and aurorae usually follow the 11-year cycle of sunspot activity, but 1645 to 1715 saw the ‘Maunder Minimum’, with ‘only a sprinkling’ of sunspots in the whole period. For the duration of it the northern hemisphere of the Earth had a mini-ice-age, with winter ice fairs on the Thames, and between 1400 and 1450 there was a similar drop now called ‘the Spörer Minimum’, during which the Viking colonies in Greenland were wiped out by the cold and the Inuit were forced to abandon a 4000-year old colony on Ellesmere Island, northern Canada, originally established from Mongolia.

Carbon-14 deposition in tree rings increased between 1640 and 1720, so we can use earlier tree rings to chart the Sun’s previous activity, and the record now goes back about 11,000 years. The rise in the 20th century corresponds to a marked increase in the numbers of solar flares up to 1970, after which it flattened out – in the run-up to the 2001 peak, sunspots in 1999 were well below predicted levels, and the most recent peak was even lower. The most intense solar activity since the Bronze Age was from 1150-1200 AD, peaking in the 1170’s with particularly warm weather. In 1186, for example, astrologers predicted storms and pestilence because of a planetary conjunction in Libra, “the season proving, in a more than usual degree, serene and benignant”. In North America, however, the solar activity may have caused the steep temperature rise and drought which forced the Anasazi people to abandon their rock dwellings in the Grand Canyon at that time.

It takes four minutes for the Earth’s rotation to change the position of the Sun in the sky by a single degree, but awareness of the alternation of day and night is programmed into us at a very basic level – in isolation, without external clues, human beings tend to drift towards a twenty-hour rhythm which goes back six hundred million years, to the time when life crawled from the sea on to the land. It is hard to say whether the dominant factor was the day/night cycle or the ebb and flow of the tides. But it is the braking effect of the tides which has slowed down the Earth since then, and it is the perception of the day/night cycle which resets our biological clocks.

### The Ecliptic

The plane of the Earth’s orbit around the Sun is the Ecliptic, the center line of the Zodiac as the Sun moves along it over the course of the year, its horizon position varies from its most southerly midwinter rise and set, when it’s overhead at the Tropic of Capricorn, to its most northerly midsummer rise and set, when it’s overhead at the Tropic of Cancer. The Stonehenge Avenue and the later structure both mark the midsummer sunrise.

The evidence suggests that the beginnings of agriculture did not come directly from the annual cycle of plants but from the movements of the great herds of game. From moving with the herds, as the Lapps do even today, domestication and nomadic herding comprised the next step and led to the first attempts at agriculture and fixed settlements. The very oldest towns, such as Jericho (c.8000 BC), came before the first crop-farming. The move to an annual cycle obviously required a true calendar, even if none had previously been attempted. But the year is harder to calibrate than the month – even today, a week is a long time in politics – and most societies, if not all, tried to fit their lunar calendar into the year. The cause is a lost one – the lunar and solar cycles are not commensurate i.e., they do not fit together in any straightforward numerical relationship – and different cultures made different compromises to divide the year into approximate “months” of convenient length, while keeping religious ritual and agricultural practice in step with the solar year.

### The Zodiacal Light

The Zodiacal Light is a cone which appears above the rising and setting Sun, best seen from the Tropics where twilight is short. On the opposite side of the Earth from the Sun keen-eyed observers sometimes glimpse a related glow called the Gegenschein (counter-glow). The Zodiacal Light is faint and the Gegenschein much fainter. Until the space age it wasn’t known whether the two effects were truly in interplanetary space or generated by a dust cloud surrounding the Earth, but the Pioneer 10 space probe found that both effects persisted as far out as the Asteroid Belt. The dust comes partly from there, and part of it is released by comets passing through the inner Solar System, and it spirals towards the Sun due to the Poynting-Robertson effect, in which light from the Sun exercises a slight but significant braking effect.

Artists often depict the Sun in space surrounded by the corona and the Zodiacal Light, all seen at once. The Zodiacal Light is seldom seen from Earth except in the Tropics, and then only after sunset or before sunrise, while to make the corona visible requires an occulting object in space – either the disc of the Moon, in a solar eclipse, or an occulting disc within a telescope like the ones on Skylab in 1973 and now on SOHO, currently between Earth and Sun at the L1 point. (See ‘New Discoveries…’ below.)

### Helium

The element helium is named after helios, the Sun, because it was first discovered in the spectrum of sunlight, then in natural vents in Texas and Kansas (the first helium-filled airship was the US Shenandoah). The element exists in two isotopes with very different properties, both potentially useful to high-tech civilisations.

Helium-4 exists on Earth in small quantities, released by radioactive decay. In liquid form it’s the coldest substance in the Universe, only just above Absolute Zero. It has negative surface tension, so it will climb out of an open-topped container and flow down the sides it has superfluidity, so you can pump it both ways along the same pipe at the same time and when used as a refrigerant it promotes superconductivity, reducing the resistance of electrical conductors to zero. So it has many possible uses, for instance in power transmission and many systems requiring high-energy magnetic fields, including radiation shielding for manned spacecraft.

The superfluidity of liquid helium II was discovered, named and explained by Peter Leonidovich Kapitza (1894-1984), who did so at Cambridge in 1930-37 (Simon Mitton, “Fred Hoyle, A Life in Science”, Aurum, 2005). Kapitza received the Nobel Prize in 1978. Outside the Sun the largest repository of helium is in the atmosphere of Jupiter, and Isaac Asimov suggested that helium would be the planet’s major export to the Solar System. (‘The Element of Perfection’ in “View from a Height”, Dobson, 1964).

However the lighter helium-3 may prove to be even more important. If we ever master controlled fusion, for energy generation or spaceship propulsion, the most promising reaction seems to be the fusion of deuterium (heavy hydrogen) with helium-3. The theory was examined in detail by the British Interplanetary Society’s interstellar probe study, “Project Daedalus” (BIS, 1978). Deuterium is plentiful on Earth, particularly in sea-water, and Helium-3 is found in small quantities in solar wind deposits on the lunar soil, but claims that it could solve the USA’s energy problems seem highly questionable. To meet even 10% of the US energy requirement, so much lunar soil would have to be strip-mined that the scar would become visible from Earth with the naked eye, in only three years! (“America at the Threshold”, US Goverment Printing Office, 1991.)

Helium-3 can be manufactured in nuclear reactors, but that would generate so much waste energy that the plant would have to be on the Farside of the Moon to protect the Earth! Extracting it from Jupiter would be much more practicable, and I outlined ways to do it in my books Man and the Planets and Incoming Asteroid!, as well as in several scientific papers and articles.

### New Discoveries under the Sun – and on it – and over it.

After a decade of discoveries pouring in from spacecraft orbiting Mercury, Venus, the Moon, Mars, Vesta, Ceres and Saturn, with the Juno mission now orbiting Jupiter and the Curiosity and Perseverance rovers hard at work on Mars, and Lunar Reconnaissance Orbiter still circling the Moon, it’s easy to forget that there’s a flotilla of spacecraft dedicated to observing the Sun. Among them Europe’s veteran SOHO mission remains on-station at the Sun-Earth L1 point, between us and the Sun, watching what happens on it, what comes off from it and giving early warning of what’s coming our way, including the violent Coronal Mass Ejections which cause aurorae and other disturbances of Earth’s magnetic field. The Japanese Hinode, launched in 2006, is still hard at work. STEREO, a twin mission launched in 2006, sent probes to opposite sides of the Sun to give us 24/7 coverage of all events on it, and one of them is still operational.

Not that it should be necessary, but STEREO finally laid to rest the myth of a planet twinned with Earth, orbiting on the far side of the Sun, featured in a novel by Edgar Wallace, the feminist ‘Twin Earths’ comic strip of the 1950s, and in the ‘Gor’ novels of John Norman, diametrically opposite in sentiment as well as location. Relative to Earth the planet would have been at the Sun-Earth L3 point, which is a condition of unstable equilibrium: the pulls of the other planets would quickly have brought the ‘counter-Earth’ into view.

More importantly, the images from SOHO and the other probes are complemented by higher-resolution ones from SDO, the Solar Dynamics Observer launched in February 2010. Big discoveries are still being made: for the first time scientists have been able to see the predicted Alfvén waves in spicules projecting from the surface of the Sun, compared to the action of the wind on stalks of wheat. Not only can they now be seen, but they prove to carry so much energy that they can explain the heating of the corona, the Sun’s outer atmosphere, which has been a mystery for decades – and also the energy of the Solar Wind, which can reach speeds of 1.5 million miles per hour as it streams through holes in the corona, discovered by the Apollo Telescope Mount on the Skylab space station in 1973. (Tammy Plotner, ‘Amber Waves Of Energy’, Universe Today, July 29, 2011.)

SOHO’s camera resolution is high enough to see seismic waves on the surface of the Sun, and a great deal has been discovered from them about the behaviour of the Sun and other stars. Combining data from SDO and from SOHO, a team at Stanford University have computed the behaviour of seismic waves below the solar surface and discovered that they can detect disturbances 60,000 km below the surface, travelling upwards at 1,000 to 2,000 kilometres per hour, which evolve into huge vortices as they reach the surface – sunspot groups. It allows their outbreaks to be predicted one to three days in advance. (Tammy Plotner, ‘Scientists Detect Sunspots Before They Emerge’, Universe Today, August 23, 2011.)

At the same time as we’re explaining the newest features of the Sun to be discovered, we’re finding the origins of the ones which have been known for longest. The first naked-eye sunspot sighting on record is prior to 800 BC, in the Chinese Book of Changes, and the first known drawing is by John of Worcester in 1128-29. (Chris Kitchin, ‘Rhythms of the Sun’, Astronomy Now, November 2001.) For as long as we have satellites like SOHO, STEREO and SDO operational, there’s now no danger that astronauts on the Moon, unprotected by the Earth’s magnetic field from last to first quarter, can be surprised by sunspots and flares coming round the Sun as they did during the Apollo 16 mission, fortunately without the particle streams coming this way.

## Cool new worlds found in our cosmic backyard

How complete is our census of the Sun's closest neighbors? Astronomers using NSF's NOIRLab facilities and a team of data-sleuthing volunteers participating in Backyard Worlds: Planet 9, a citizen science project, have discovered roughly 100 cool worlds near the Sun -- objects more massive than planets but lighter than stars, known as brown dwarfs. Several of these newly discovered worlds are among the very coolest known, with a few approaching the temperature of Earth -- cool enough to harbor water clouds.

Discovering and characterizing astronomical objects near the Sun is fundamental to our understanding of our place in, and the history of, the Universe. Yet astronomers are still unearthing new residents of the Solar neighborhood. A remarkable breakthrough was announced today, with the discovery of roughly 100 cool brown dwarfs near the Sun [1].The new Backyard Worlds discoveries bridge a previously empty gap in the range of low-temperature brown dwarfs, identifying a long-sought missing link within the brown dwarf population.

"These cool worlds offer the opportunity for new insights into the formation and atmospheres of planets beyond the Solar System," said Aaron Meisner from the National Science Foundation's NOIRLab and the lead author of the research paper. "This collection of cool brown dwarfs also allows us to accurately estimate the number of free-floating worlds roaming interstellar space near the Sun."

This major advancement was made possible with archival data from the Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory (KPNO) and the Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO), which were made available through the Community Science and Data Center (CSDC), all programs of NSF's NOIRLab. Large survey data sets were then made available to the Backyard Worlds volunteers using NOIRLab's Astro Data Lab science platform. The results, to be published in TheAstrophysical Journal, demonstrate the rapidly growing role of survey and archival data research in astronomy today.

Brown dwarfs lie somewhere between the most massive planets and the smallest stars. Lacking the mass needed to sustain nuclear reactions in their core, brown dwarfs resemble cooling embers. Their low mass, low temperature and lack of internal nuclear reactions make them extremely faint -- and therefore extremely difficult to detect. Because of this, when searching for the very coolest brown dwarfs, astronomers can only hope to detect such objects relatively close to the Sun.

To help find our Sun's coldest and nearest neighbors, the astronomers of the Backyard Worlds project turned to a worldwide network of more than 100,000 citizen scientists [2]. These volunteers diligently inspect trillions of pixels of telescope images to identify the subtle movements of brown dwarfs and planets. Despite the abilities of machine learning and supercomputers, there's no substitute for the human eye when it comes to scouring telescope images for moving objects.

The keen eyes of the Backyard Worlds volunteers have already discovered more than 1,500 cold worlds near to the Sun, and today's paper presents roughly 100 of the coldest in that sample. According to Meisner, this is a record for any citizen science program by a factor of about 20, and 20 citizen scientists are listed as co-authors of the study. A handful of these cool worlds -- which are among the very coldest brown dwarfs known -- approach the temperature of Earth. NASA's Spitzer Space Telescope provided the brown dwarf temperature estimates [3].

Brown dwarfs are expected to cool as they age, passing from near-stellar temperatures down to planetary temperatures and below, fading all the while and eventually winking out. The new discoveries attest to this picture by uncovering elusive examples of brown dwarfs approaching Earth-temperature.

"This paper is evidence that the solar neighborhood is still uncharted territory and citizen scientists are excellent astronomical cartographers," said co-author Jackie Faherty of the American Museum of Natural History. "Mapping the coldest brown dwarfs down to the lowest masses gives us key insights into the low-mass star formation process while providing a target list for detailed studies of the atmospheres of Jupiter analogs."

Citizen scientist, Astro Data Lab user, and paper co-author Jim Walla added, "It's awesome to know that our discoveries are now counted among the Sun's neighbors and will be targets of further research."

Alongside the dedicated efforts of the Backyard Worlds volunteers, NOIRLab's Astro Data Lab was instrumental in this research. The technical burden of downloading billion-object astronomical catalogs is typically insurmountable for individual investigators -- including most professional astronomers. "AstroData Lab's open and accessible web portal allowed Backyard Worlds citizen scientists to easily query massive catalogs for brown dwarf candidates," explained NOIRLab astronomer Stephanie Juneau, who helped introduce the citizen scientists to Astro Data Lab. Astro Data Lab also enables convenient matching between data sets from NOIRLab telescopes and external facilities, such as NASA's WISE satellite, that jointly contributed to these brown dwarf discoveries.

In addition to Astro Data Lab's making data accessible to the Backyard Worlds collaboration, archival observations by telescopes at two other NOIRLab Programs -- CTIO and KPNO -- were also key to this discovery. "Wide-area imaging from NOIRLab's Mayall and Blanco telescopes was also critical," explained Aaron Meisner. "To select only the very coldest brown dwarfs, we inspected deep images from a variety of sensitive astronomical surveys."

"It's great to see such thrilling results from NOIRLab's efforts to broaden participation in astronomy research," said Chris Davis of the National Science Foundation, the US agency that supports operations at the Kitt Peak and Cerro Tololo observatories and at CSDC. "By making archival data from NSF's Mayall and Blanco telescopes publicly available and easily accessible through CSDC, folks with a fascination for astronomy can make a real contribution to science and to our understanding of the Universe."

The approach of the Backyard Worlds project -- searching for rare objects in large data sets -- is also one of the goals for the upcoming Vera C. Rubin Observatory [4]. Currently under construction on Cerro Pachón in the Chilean Andes, Rubin Observatory will image the visible sky from the southern hemisphere every three nights over ten years, providing a vast amount of data that will enable new ways of doing astrophysical research.

"Vast modern data sets can unlock landmark discoveries, and it's exciting that these could be spotted first by a citizen scientist," concludes Aaron Meisner. "These Backyard Worlds discoveries show that members of the public can play an important role in reshaping our scientific understanding of our solar neighborhood."

[1] The closest of these new discoveries is roughly 23 light-years away from the Sun. Many more of these brown dwarfs are in the 30-60 light-year distance range.

[2] Backyard Worlds: Planet 9 is hosted by Zooniverse.

[3] Complementary follow-up observations were also supplied by Keck Observatory, Mont Mégantic Observatory, and Carnegie Institution for Science's Las Campanas Observatory.

[4] Rubin Observatory and Department of Energy (DOE) Legacy Survey of Space and Time Camera are operated by NSF's NOIRLab and SLAC National Accelerator Laboratory (SLAC).