Is there a stellar database that indicates how long ago stars in our Galaxy formed?

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There are several ways of determining the age of a star: its position in the HR diagram, the presence of a protoplanetary disk, it belonging to a cluster…

When did the stars in our Galaxy form? Do we know their age? Does there exists a database of stars for which we know the age, from which this average could be computed ? Is there a histogram of how long ago the stars in our Galaxy formed?

The amount of stars being formed as a function of time is shown for instance in Snaith et al 2014, but this doesn't directly translate into a histogram of the age of the stars currently in our galaxy, since the more massive ones that formed early on are gone.

There isn't really a database as you request. Finding the ages of stars is difficult. Only one star has an accurately known age - the Sun. That comes from radioisotope dating of meteorites. For other stars we must rely on models to a greater or lesser extent and we can only estimate an age if the star has a mass or is in a phase of its evolution where things are changing rapidly enough to give some handle on how old it is. For stars at the mass of the Sun or a little bit bigger, one can use evolution in the Hertzsprung-Russell diagram. Stars become more luminous as they burn through their core hydrogen and precise measurements can give an age to about $$pm 1$$ Gyr. These are the data that Snaith et al. (2015) use in their paper.

So what Snaith et al. do is they attempt to constrain the star formation history of our Galaxy (or at least stars in the disk of our Galaxy) by making a Galactic Chemical Evolution model that predicts how the abundances of silicon and iron change with time and compare the results of their model with the observed silicon and iron abundances in this set of solar-type stars with reasonably well-estimated ages. Iron and silicon are used because they provide differing constraints. Iron is produced during the evolution of relatively low-mass ($$1.5 - 4 M_{odot}$$) stars that live their long(ish) lives (1-10 Gyr), become white dwarfs and then some of the white dwarfs explode as type Ia supernovae. Silicon is produced by massive stars with short lives ($$<0.1$$ Gyr) and spread promptly into the interstellar medium by core-collape type II supernovae.

A representative lot from Snaith's paper is shown below. The star forming rate is shown in plot (a) and the match of the model to the observational data (they are trying to match the solid green points here) on chemical abundance is shown in the other plots.

They test a number of models and different assumptions, but it appears to be quite a robust result (found in other studies also) that there was a burst of star formation 10-12 billion years ago, a lull at 8-9 billion years ago and then a more constant, lower rate over the last 7 billion years. The very low rate in their model over the last 2 billion years is unlikely to be true and is probably an artefact of the age uncertainties in their data points (e.g. stars with age of $$1 pm 1$$ billion years) and we certainly see plenty of star formation in the disk of the Galaxy today.

Contrary to what you say in your question this does (almost) represent the age distribution of stars in the Galaxy. That is because the vast majority of stars (90+%) are of a solar mass or lower. Such stars have main sequence life times of 11+ billion years, such that almost every one of them that was born is still a main sequence star now.

There will be a small reduction in the numbers of the oldest stars, because a small fraction of those ($$<10$$%) will have lived and died and also because a small fraction of the oldest stars may have escaped the Galaxy entirely (due to various kinematic processes that cause their orbital speeds to change). There is also an effect whereby if you take a volume close to the Sun, the oldest stars will be under-represented because they are distributed more broadly around the disc midplane for the same reason.

But by an large, the upper left plot in the picture is roughly what we expect the age distribution of stars to be in the Galactic disk.

There are a small population of very old stars ($$12+$$ bilion years), perhaps 1%, that are distributed more widely in the Galactic halo.

I don't know about stars in general, but for giant stars, the following histogram shows the age distribution of those in our Galaxy based on their location in the Galaxy.

This means that most stars are between 7 and 8 billion years old.

In terms of asking the question in the title, there are various catalogues which include stellar ages, if you search on VizieR for the category "Ages" (in the "Astronomy" menu on the right-hand side) you will find a large number of such catalogues, but you will have to bear in mind that they focus on certain sets of objects rather than stars in general.

One example is the Geneva-Copenhagen Survey of the Solar Neighbourhood (which includes F- and G-dwarfs), which has also been used as the source for stellar ages in the XHIP (Extended Hipparcos compilation) catalogue.

Another example is the Lachaume et al. (1999) age determinations for main-sequence stars in the B9-K9 spectral type range.

Computing the age distribution of stars in the galaxy from these catalogues will require correcting for the various selection biases involved, this is not a trivial task.

Where do new stars form in galaxies?

An optical image of the spiral galaxy NGC 300 with molecular clouds shown in blue. An analysis of star formation in these clouds show that the first stars that form quickly disperse the cloud, stifling further star formation. (Image courtesy of Diederik Kruijssen & Nature)

Spiral galaxies like our own Milky Way are studded with cold clouds of hydrogen gas and dust, like chocolate chips in a loaded Toll House cookie.

Astronomers have long focused on these so-called molecular clouds, suspecting that they are hotspots for star formation. But are they?

After a thorough analysis of the molecular clouds in a nearby spiral galaxy, an international team of astronomers has found that, while star formation starts up rapidly in these clouds, the newly formed stars quickly disperse the cloud – in as little as a few million years – stopping further star formation. So while star formation in cold molecular clouds is fast, it’s highly inefficient.

The findings by a collaboration led by Diederik Kruijssen from Heidelberg University will help astronomers understand where and when stars form in galaxies, which in turn determines how galaxies change over their lifetimes.

“The link between star formation and the evolution of galaxies is one of the main outstanding issues in astronomy,” said UC Berkeley postdoctoral fellow Anna McLeod, co-author of a paper published this week in Nature describing the analysis. “How do stars form within the galactic context? What is their role in shaping the evolution of the galaxy they formed in? And on what timescales does this all happen?”

The results come from use of a novel statistical approach that the team applied to data from the nearby spiral galaxy NGC 300, which is about 6 million light years from Earth in the direction of the constellation Sculptor. The analysis showed that the intense radiation and stellar winds emitted by the young, massive stars forming in these clouds tamp down the formation of new generations of stars.

“The intense radiation from young stars disperses their parent molecular cloud by heating them and blowing hot bubbles of interstellar gas,” said co-author Mélanie Chevance, also from Heidelberg University. “This way, only two to three percent of the mass in molecular clouds is actually converted into stars.”

The image on the left shows that the positions of molecular clouds (blue) and emission from young stars (pink) do not coincide on small spatial scales. The two branches on the right quantify this displacement by showing that molecular clouds and young stars are only correlated when averaging over a large part of the galaxy, corresponding to about 3,000 lightyears. (Image courtesy of Diederik Kruijssen & Nature)

“Molecular clouds in NGC300 live for about 10 million years, and take only about 1.5 million years to be destroyed, well before the most massive stars have reached the end of their lives and explode as supernovae,” added astrophysicist Kruijssen.

As a result, these molecular clouds are short-lived structures with rapid lifecycles, making galaxies “cosmic cauldrons” constantly changing their appearance.

The new analysis makes use of archival observational data in one single optical wavelength. McLeod is the principal investigator of a project to analyze a new, large observational dataset of NGC 300 that will allow the team to apply this novel statistical method to other optical wavelengths so as to capture star formation at many different evolutionary stages.

“We are now entering the era in which we can map many, many galaxies, near and far, at many different wavelengths simultaneously via so-called integral field spectroscopy,” McLeod said. “We can then apply this new statistical method to these truly huge datasets and systematically understand star formation across the vast galaxy zoo that is out there.”

UCI celestial census indicates that black holes pervade the universe

Irvine, Calif., Aug. 7, 2017 - After conducting a cosmic inventory of sorts to calculate and categorize stellar-remnant black holes, astronomers from the University of California, Irvine have concluded that there are probably tens of millions of the enigmatic, dark objects in the Milky Way - far more than expected.

"We think we've shown that there are as many as 100 million black holes in our galaxy," said UCI chair and professor of physics & astronomy James Bullock, co-author of a research paper on the subject in the current issue of Monthly Notices of the Royal Astronomical Society.

UCI's celestial census began more than a year and a half ago, shortly after the news that the Laser Interferometer Gravitational-Wave Observatory, or LIGO, had detected ripples in the space-time continuum created by the distant collision of two black holes, each the size of 30 suns.

"Fundamentally, the detection of gravitational waves was a huge deal, as it was a confirmation of a key prediction of Einstein's general theory of relativity," Bullock said. "But then we looked closer at the astrophysics of the actual result, a merger of two 30-solar-mass black holes. That was simply astounding and had us asking, 'How common are black holes of this size, and how often do they merge?'"

He said that scientists assume most stellar-remnant black holes - which result from the collapse of massive stars at the end of their lives - will be about the same mass as our sun. To see evidence of two black holes of such epic proportions finally coming together in a cataclysmic collision had some astronomers scratching their heads.

UCI's work was a theoretical investigation into the "weirdness of the LIGO discovery," Bullock said. The research, led by doctoral candidate Oliver Elbert, was an attempt to interpret the gravitational wave detections through the lens of what is known about galaxy formation and to form a framework for understanding future occurrences.

"Based on what we know about star formation in galaxies of different types, we can infer when and how many black holes formed in each galaxy," Elbert said. "Big galaxies are home to older stars, and they host older black holes too."

According to co-author Manoj Kaplinghat, UCI professor of physics & astronomy, the number of black holes of a given mass per galaxy will depend on the size of the galaxy.

The reason is that larger galaxies have many metal-rich stars, and smaller dwarf galaxies are dominated by big stars of low metallicity. Stars that contain a lot of heavier elements, like our sun, shed a lot of that mass over their lives. When it comes time for one to end it all in a supernova, there isn't as much matter left to collapse in on itself, resulting in a lower-mass black hole. Big stars with low metal content don't shed as much of their mass over time, so when one of them dies, almost all of its mass will wind up in the black hole.

"We have a pretty good understanding of the overall population of stars in the universe and their mass distribution as they're born, so we can tell how many black holes should have formed with 100 solar masses versus 10 solar masses," Bullock said. "We were able to work out how many big black holes should exist, and it ended up being in the millions - way more than I anticipated."

In addition, to shed light on subsequent phenomena, the UCI researchers sought to determine how often black holes occur in pairs, how often they merge, and how long it takes. They wondered whether the 30-solar-mass black holes detected by LIGO were born billions of years ago and took a long time to merge or came into being more recently (within the past 100 million years) and merged soon after.

"We show that only 0.1 to 1 percent of the black holes formed have to merge to explain what LIGO saw," Kaplinghat said. "Of course, the black holes have to get close enough to merge in a reasonable time, which is an open problem."

Elbert said he expects many more gravitation wave detections so that he and other astronomers can determine if black holes collide mostly in giant galaxies. That, he said, would tell them something important about the physics that drive them to coalesce.

According to Kaplinghat, they may not have to wait too long, relatively speaking. "If the current ideas about stellar evolution are right, then our calculations indicate that mergers of even 50-solar-mass black holes will be detected in a few years," he said.

About the University of California, Irvine: Founded in 1965, UCI is the youngest member of the prestigious Association of American Universities. The campus has produced three Nobel laureates and is known for its academic achievement, premier research, innovation and anteater mascot. Led by Chancellor Howard Gillman, UCI has more than 30,000 students and offers 192 degree programs. It's located in one of the world's safest and most economically vibrant communities and is Orange County's second-largest employer, contributing \$5 billion annually to the local economy. For more on UCI, visit http://www. uci. edu.

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Star cluster surrounds wayward black hole in cannibal galaxy

IMAGE: This spectacular edge-on galaxy, called ESO 243-49, is home to an intermediate-mass black hole that may have been stripped off of a cannibalized dwarf galaxy. The estimated 20,000-solar-mass black hole. view more

Credit: NASA ESA and S. Farrell, Sydney Institute for Astronomy, University of Sydney

Astronomers using NASA's Hubble Space Telescope may have found evidence for a cluster of young, blue stars encircling one of the first intermediate-mass black holes ever discovered. Astronomers believe the black hole may once have been at the core of a now-disintegrated unseen dwarf galaxy. The discovery of the black hole and the possible star cluster has important implications for understanding the evolution of supermassive black holes and galaxies.

Astronomers know how massive stars collapse to form black holes but it is not clear how supermassive black holes, which can weigh billions of times the mass of our sun, form in the cores of galaxies. One idea is that supermassive black holes may build up through the merger of smaller black holes.

Sean Farrell of the Sydney Institute for Astronomy in Australia discovered a middleweight black hole in 2009 using the European Space Agency's XMM-Newton X-ray space telescope. Known as HLX-1 (Hyper-Luminous X-ray source 1), the black hole has an estimated weight of about 20,000 solar masses. It lies towards the edge of the galaxy ESO 243-49, 290 million light-years from Earth.

Farrell then observed HLX-1 simultaneously with NASA's Swift observatory in X-ray and Hubble in near infrared, optical and ultraviolet wavelengths. The intensity and the color of the light may indicate the presence of a young, massive cluster of blue stars, perhaps 250-light-years across, encircling the black hole. Hubble can't resolve the stars individually because the suspected cluster is too far away. The brightness and color is consistent with other clusters of stars seen in other galaxies, but some of the light may be coming from the gaseous disk around the black hole.

"Before this latest discovery, we suspected that intermediate-mass black holes could exist, but now we understand where they may have come from," Farrell said. "The fact that there seems to be a very young cluster of stars indicates that the intermediate-mass black hole may have originated as the central black hole in a very-low-mass dwarf galaxy. The dwarf galaxy might then have been swallowed by the more massive galaxy, just as happens in our Milky Way."

From the signature of the X-rays, Farrell's team knew there would be some blue light emitted from the high temperature of the hot gas in the disk swirling around the black hole. They couldn't account for the red light coming from the disk. It would have to be produced by a much cooler gas, and they concluded this would most likely come from stars. The next step was to build a model that added the glow from a population of stars. These models favor the presence of a young massive cluster of stars encircling the black hole, but this interpretation is not unique, so more observations are needed. In particular, the studies led by Roberto Soria of the Australian International Centre for Radio Astronomy Research, using data from Hubble and the ground-based Very Large Telescope, show variations in the brightness of the light that a star cluster couldn't cause. This indicates that irradiation of the disk itself might be the dominant source of visible light, rather than a massive star cluster.

"What we can definitely say with our Hubble data is that we require both emission from an accretion disk and emission from a stellar population to explain the colors we see," said Farrell.

Such young clusters of stars are commonly found inside galaxies like the host galaxy, but not outside the flattened starry disk, as found with HLX-1. One possible scenario is that the HLX-1 black hole was the central black hole in a dwarf galaxy. The larger host galaxy may then have captured the dwarf. In this conjecture, most of the dwarf's stars would have been stripped away through the collision between the galaxies. At the same time, new, young stars would have formed in the encounter. The interaction that compressed the gas around the black hole would then have also triggered star formation.

Farrell theorizes that the possible star cluster may be less than 200 million years old. This means that the bulk of the stars formed following the dwarf's collision with the larger galaxy. The age of the stars tells how long ago the two galaxies crashed into each other.

Farrell proposed for more observations this year. The new findings are published in the February 15 issue of the Astrophysical Journal . Soria and his colleagues have published their alternative conclusions in the January 17 online issue of the Monthly Notices of the Royal Astronomical Society .

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Md., manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Md., conducts Hubble science operations. STScI is operated by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.

For more images and information about HLX-1, visit:

Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.

UCI celestial census indicates that black holes pervade the universe

There are a lot more black holes in the Milky Way than previously thought, according to a new UCI study by (from left) James Bullock, chair and professor of physics & astronomy Manoj Kaplinghat, professor of physics & astronomy and Oliver Elbert, physics & and astronomy graduate student. Credit: Steven Zylius / UCI

After conducting a cosmic inventory of sorts to calculate and categorize stellar-remnant black holes, astronomers from the University of California, Irvine have concluded that there are probably tens of millions of the enigmatic, dark objects in the Milky Way - far more than expected.

"We think we've shown that there are as many as 100 million black holes in our galaxy," said UCI chair and professor of physics & astronomy James Bullock, co-author of a research paper on the subject in the current issue of Monthly Notices of the Royal Astronomical Society.

UCI's celestial census began more than a year and a half ago, shortly after the news that the Laser Interferometer Gravitational-Wave Observatory, or LIGO, had detected ripples in the space-time continuum created by the distant collision of two black holes, each the size of 30 suns.

"Fundamentally, the detection of gravitational waves was a huge deal, as it was a confirmation of a key prediction of Einstein's general theory of relativity," Bullock said. "But then we looked closer at the astrophysics of the actual result, a merger of two 30-solar-mass black holes. That was simply astounding and had us asking, 'How common are black holes of this size, and how often do they merge?'"

He said that scientists assume most stellar-remnant black holes - which result from the collapse of massive stars at the end of their lives - will be about the same mass as our sun. To see evidence of two black holes of such epic proportions finally coming together in a cataclysmic collision had some astronomers scratching their heads.

UCI's work was a theoretical investigation into the "weirdness of the LIGO discovery," Bullock said. The research, led by doctoral candidate Oliver Elbert, was an attempt to interpret the gravitational wave detections through the lens of what is known about galaxy formation and to form a framework for understanding future occurrences.

"Based on what we know about star formation in galaxies of different types, we can infer when and how many black holes formed in each galaxy," Elbert said. "Big galaxies are home to older stars, and they host older black holes too."

According to co-author Manoj Kaplinghat, UCI professor of physics & astronomy, the number of black holes of a given mass per galaxy will depend on the size of the galaxy.

The reason is that larger galaxies have many metal-rich stars, and smaller dwarf galaxies are dominated by big stars of low metallicity. Stars that contain a lot of heavier elements, like our sun, shed a lot of that mass over their lives. When it comes time for one to end it all in a supernova, there isn't as much matter left to collapse in on itself, resulting in a lower-mass black hole. Big stars with low metal content don't shed as much of their mass over time, so when one of them dies, almost all of its mass will wind up in the black hole.

"We have a pretty good understanding of the overall population of stars in the universe and their mass distribution as they're born, so we can tell how many black holes should have formed with 100 solar masses versus 10 solar masses," Bullock said. "We were able to work out how many big black holes should exist, and it ended up being in the millions - way more than I anticipated."

In addition, to shed light on subsequent phenomena, the UCI researchers sought to determine how often black holes occur in pairs, how often they merge, and how long it takes. They wondered whether the 30-solar-mass black holes detected by LIGO were born billions of years ago and took a long time to merge or came into being more recently (within the past 100 million years) and merged soon after.

"We show that only 0.1 to 1 percent of the black holes formed have to merge to explain what LIGO saw," Kaplinghat said. "Of course, the black holes have to get close enough to merge in a reasonable time, which is an open problem."

Elbert said he expects many more gravitation wave detections so that he and other astronomers can determine if black holes collide mostly in giant galaxies. That, he said, would tell them something important about the physics that drive them to coalesce.

According to Kaplinghat, they may not have to wait too long, relatively speaking. "If the current ideas about stellar evolution are right, then our calculations indicate that mergers of even 50-solar-mass black holes will be detected in a few years," he said.

Is there a stellar database that indicates how long ago stars in our Galaxy formed? - Astronomy

A galaxy is a huge collection of stellar and interstellar matter isolated in space and bound together by its own gravity. Because we live within it, the Galactic disk of our own Milky Way Galaxy appears as a broad band of light across the sky, a band called the Milky Way. Near the center, the Galactic disk thickens into the Galactic bulge. The disk is surrounded by a roughly spherical Galactic halo of old stars and star clusters. Our Galaxy, like many others visible in the sky, is a spiral galaxy.

The halo can be studied using variable stars, whose luminosity changes with time. Pulsating variable stars vary in brightness in a repetitive and predictable way. Two types of pulsating variable stars of great importance to astronomers are RR Lyrae variables and Cepheid variables, whose characteristic light curves make them easily recognizable. All RR Lyrae stars have roughly the same luminosity. Astronomers can determine the luminosity of Cepheids by measuring the pulsation period and using the period—luminosity relationship, a simple correlation between period and absolute brightness. The brightest Cepheids can be seen at distances of millions of parsecs, extending the cosmic distance ladder well beyond our own Galaxy. RR Lyrae stars are fainter but much more numerous, making them very useful within the Milky Way.

In the early twentieth century, Harlow Shapley used RR Lyrae stars to determine the distances to many of the Galaxy's globular clusters. He found that the clusters have a roughly spherical distribution in space, but the center of the sphere lies far from the Sun. The globular clusters are now known to map out the true extent of the luminous portion of the Milky Way Galaxy. The center of their distribution is close to the Galactic center, which lies about 8 kpc from the Sun.

Disk and halo stars differ in their spatial distributions, ages, colors, and orbital motion. The luminous portion of our Galaxy has a diameter of about 30 kpc. The halo lacks gas and dust, so no stars are forming there. All halo stars are old. The gas-rich disk is the site of current star formation and contains many young stars. Stars and gas within the Galactic disk move on roughly circular orbits around the Galactic center. Stars in the halo and bulge move on largely random three-dimensional orbits that pass repeatedly through the disk plane but have no preferred orientation. Halo stars appeared early on, before the Galactic disk took shape, when there was still no preferred orientation for their orbits. As the gas and dust formed a rotating disk, stars that formed in the disk inherited its overall spin and so moved on circular orbits in the Galactic plane, as they do today.

In the vicinity of the Sun the Galactic disk is about 300 pc thick. Young stars, gas, and dust are more narrowly confined older stars have a broader distribution. Intermediate between the young disk and the old halo, in both age and spatial distribution, are the stars of the thick disk, which is about 2-3 kpc thick.

Attempts to map out the Galactic disk by optical observations are defeated by interstellar absorption. Astronomers use radio observations to explore the Galactic disk because radio waves are largely unaffected by interstellar dust. Regions where most of the hydrogen is in atomic form may be studied using 21-cm radiation. Regions where the gas is mostly molecular are studied through radio molecular emission lines. Gas has been detected in the disk at up to 50 kpc from the Galactic center. Regions where the gas is mostly molecular are usually studied by observing radio emission lines from "tracer" molecules, such as carbon monoxide. The gas distribution fattens near the center into the Galactic bulge. Radio-emitting gas has been detected in the disk at up to 50 kpc from the Galactic center.

Radio observations clearly reveal the extent of our Galaxy's spiral arms. The spiral arms in spiral galaxies are regions of the densest interstellar gas and are the places where star formation is taking place. The spirals cannot be "tied" to the disk material, as the disk's differential rotation would have wound them up long ago. Instead, they may be spiral density waves that move through the disk, triggering star formation as they pass by. Alternatively, the spirals may arise from self-propagating star formation, when shock waves produced by the formation and evolution of one generation of stars triggers the formation of the next.

The Galactic rotation curve plots the orbital speed of matter in the disk versus distance from the Galactic center. By applying Newton's laws of motion, astronomers can determine the mass of the Galaxy. They find that the Galactic mass continues to increase beyond the radius defined by the globular clusters and the spiral structure we observe. The rotation curves of our own and other galaxies show that many, if not all, galaxies have an invisible dark halo containing far more mass than the visible portion of the galaxies. The dark matter making up these dark halos is of unknown composition. Leading candidates include low-mass stars and exotic subatomic particles. Recent attempts to detect stellar dark matter have used the fact that a faint foreground object can occasionally pass in front of a more distant star, deflecting the star's light and causing its apparent brightness to increase temporarily. This deflection is called gravitational lensing.

Astronomers working at infrared and radio wavelengths have uncovered evidence of energetic activity within a few parsecs of the Galactic center. The leading explanation is that a black hole 1ר million times more massive than the Sun resides at the heart of our Galaxy.

SELF-TEST: TRUE OR FALSE?

1. Cepheids can be used to determine the distances to the nearest galaxies. (Hint)

2. RR Lyrae stars are a type of cataclysmic variable. (Hint)

3. The Galactic halo contains about as much gas and dust as the Galactic disk. (Hint)

4. The Galactic disk contains only old stars. (Hint)

5. Population I objects are found only in the Galactic halo. (Hint)

6. Up until the 1930s, the main error made in determining the size of the Galaxy was due to an incorrectly calibrated method of determining stellar distances. (Hint)

7. Astronomers use 21-cm radiation to study Galactic molecular clouds. (Hint)

8. Radio techniques are capable of mapping the entire Galaxy. (Hint)

9. In the neighborhood of the Sun, the Galaxy's spiral density wave rotates more slowly than the overall Galactic rotation. (Hint)

10. The mass of the Galaxy is determined by counting stars. (Hint)

11. Dark matter is now known to be due to large numbers of black holes. (Hint)

12. A million—solar mass black hole could account for the unusual properties of the Galactic center. (Hint)

13. Cosmic rays are very energetic photons. (Hint)

14. Most of the mass of our Galaxy exists in the form of dark matter. (Hint)

15. The Galactic center has been extensively studied at visible and ultraviolet wavelengths. (Hint)

SELF-TEST: FILL IN THE BLANK

1. One difficulty in studying our own galaxy in its entirety is that we live _____. (Hint)

2. Herschel's attempt to map the Milky Way by counting stars led to an inaccurate estimate of the Galaxy's size because he was unaware of _____. (Hint)

3. The highly flattened, circular part of the Galaxy is called the Galactic _____. (Hint)

4. The roughly spherical region of faint old stars and globular clusters in which the rest of the Galaxy is embedded is the Galactic _____. (Hint)

5. Cepheids and RR Lyrae stars are observed to vary in _____ with periods of days to months. (Hint)

6. Cepheid pulsational periods range from _____ to _____. (Hint)

7. Cepheids and RR Lyrae variables lie in a region of the H—R diagram called the _____. (Hint)

8. According to the period—luminosity relation, the longer the pulsation period of a Cepheid, the _____ its luminosity. (Hint)

9. Harlow Shapley determined the distances to the globular clusters using observations of _____. (Hint)

10. The Sun lies roughly _____ pc from the Galactic center. (Hint)

11. The orbital speed of the Sun around the Galactic center is _____. (Hint)

12. The orbits of halo objects are _____ in direction. (Hint)

13. The original cloud of gas from which the Galaxy formed probably had a size and shape similar to the present Galactic _____. (Hint)

14. Rotational velocities in the outer part of the Galaxy are _____ than would be expected on the basis of observed stars and gas, indicating the presence of _____. (Hint)

15. Observations of the _____ of infrared spectral lines indicate that gas near the Galactic center is orbiting at extremely high speeds. (Hint)

REVIEW AND DISCUSSION

1. What are spiral nebulae? How did they get that name? (Hint)

2. How are Cepheid variables used in determining distances? (Hint)

3. Roughly how far out into space can we use Cepheids to measure distance? (Hint)

4. What important discoveries were made early in this century using RR Lyrae variables? (Hint)

5. Why are the central regions of our Galaxy best studied using radio telescopes? (Hint)

6. Of what use is radio astronomy in the study of Galactic structure? (Hint)

7. Contrast the motions of disk and halo stars. (Hint)

8. Explain why Galactic spiral arms are believed to be regions of recent and ongoing star formation. (Hint)

9. Describe what happens to interstellar gas as it passes through a spiral density wave. (Hint)

10. What is self-propagating star formation? (Hint)

11. What do the red stars in the Galactic halo tell us about the history of the Milky Way? (Hint)

12. What does the rotation curve of our Galaxy tell us about its total mass? (Hint)

13. What evidence is there for that dark matter in the Galaxy? (Hint)

14. What are some possible explanations for dark matter? (Hint)

15. Why do astronomers believe that a supermassive black hole lies at the center of the Milky Way? (Hint)

PROBLEMS

1. Calculate the angular diameter of a prestellar nebula, of radius 100 A.U., lying 100 pc from Earth. Compare this with the roughly 6 ° diameter of the Andromeda galaxy (Figure 23.2a). (Hint)

2. What is the greatest distance at which an RR Lyrae star of absolute magnitude 0 could be seen by a telescope capable of detecting objects as faint as 20th magnitude? (Hint)

3. A typical Cepheid variable is 100 times brighter than a typical RR Lyrae star. How much farther away than RR Lyrae stars can Cepheids be used as distance-measuring tools? (Hint)

4. The Hubble Space Telescope can see a star like the Sun at a distance of 100,000 pc. The brightest Cepheids have luminosities 30,000 times greater than that of the Sun. How far away can HST see these Cepheids? (Hint)

5. Calculate the proper motion (in arc seconds/year) of a globular cluster with a transverse velocity (relative to the Sun) of 200 km/s and a distance of 3 kpc. Do you think that this motion is measurable? (Hint)

6. Calculate the total mass of the Galaxy lying within 20 kpc of the Galactic center if the rotation speed at that radius is 240 km/s. (Hint)

7. Using the data presented in Figure 23.16, calculate how long it takes the Sun to "lap" stars orbiting 15 kpc from the Galactic center. How long does it take matter at 5 kpc to lap us? (Hint)

8. A density wave made up of two spiral arms is moving through the Galactic disk. At the 8-kpc radius of the Sun's orbit around the Galactic center, the wave's speed is 120 km/s, and the Galactic rotation speed is 220 km/s. Calculate how many times the Sun has passed through a spiral arm since the Sun formed 4.6 billion years ago. (Hint)

9. Given the data in the previous question and the fact that O stars live at most 10 million years before exploding as supernovae, calculate the maximum distance at which an O star (orbiting at the Sun's distance from the Galactic center) can be found from the density wave in which it formed. (Hint)

10. Material at an angular distance of 0.1" from the Galactic center is observed to have an orbital speed of 1100 km/s. If the Sun's distance to the Galactic center is 8 kpc, and the material's orbit is circular and is seen edge-on, calculate the mass of the object around which the material is orbiting. (Hint)

PROJECT

1. If you are far from city lights, look for a hazy band of light arching across the sky. This is our edgewise view of the Milky Way Galaxy. The Galactic center is located in the direction of the constellation Sagittarius, highest in the sky during the summer, but visible from spring through fall. Look at the band making up the Milky Way and notice dark regions these are relatively nearby dust clouds. Sketch what you see. Look for faint fuzzy spots in the Milky Way and note their positions in your sketch. Draw in the major constellations for reference. Compare your sketch with a map of the Milky Way in a star atlas. Did you discover most of the dust clouds? Can you identify the faint fuzzy spots?

Star cluster surrounds wayward black hole in cannibal galaxy

Astronomers using NASA's Hubble Space Telescope may have found evidence for a cluster of young, blue stars encircling one of the first intermediate-mass black holes ever discovered. Astronomers believe the black hole may once have been at the core of a now-disintegrated unseen dwarf galaxy. The discovery of the black hole and the possible star cluster has important implications for understanding the evolution of supermassive black holes and galaxies.

Astronomers know how massive stars collapse to form black holes but it is not clear how supermassive black holes, which can weigh billions of times the mass of our sun, form in the cores of galaxies. One idea is that supermassive black holes may build up through the merger of smaller black holes.

Sean Farrell of the Sydney Institute for Astronomy in Australia discovered a middleweight black hole in 2009 using the European Space Agency's XMM-Newton X-ray space telescope. Known as HLX-1 (Hyper-Luminous X-ray source 1), the black hole has an estimated weight of about 20,000 solar masses. It lies towards the edge of the galaxy ESO 243-49, 290 million light-years from Earth.

Farrell then observed HLX-1 simultaneously with NASA's Swift observatory in X-ray and Hubble in near infrared, optical and ultraviolet wavelengths. The intensity and the color of the light may indicate the presence of a young, massive cluster of blue stars, perhaps 250-light-years across, encircling the black hole. Hubble can't resolve the stars individually because the suspected cluster is too far away. The brightness and color is consistent with other clusters of stars seen in other galaxies, but some of the light may be coming from the gaseous disk around the black hole.

"Before this latest discovery, we suspected that intermediate-mass black holes could exist, but now we understand where they may have come from," Farrell said. "The fact that there seems to be a very young cluster of stars indicates that the intermediate-mass black hole may have originated as the central black hole in a very-low-mass dwarf galaxy. The dwarf galaxy might then have been swallowed by the more massive galaxy, just as happens in our Milky Way."

From the signature of the X-rays, Farrell's team knew there would be some blue light emitted from the high temperature of the hot gas in the disk swirling around the black hole. They couldn't account for the red light coming from the disk. It would have to be produced by a much cooler gas, and they concluded this would most likely come from stars. The next step was to build a model that added the glow from a population of stars. These models favor the presence of a young massive cluster of stars encircling the black hole, but this interpretation is not unique, so more observations are needed. In particular, the studies led by Roberto Soria of the Australian International Centre for Radio Astronomy Research, using data from Hubble and the ground-based Very Large Telescope, show variations in the brightness of the light that a star cluster couldn't cause. This indicates that irradiation of the disk itself might be the dominant source of visible light, rather than a massive star cluster.

"What we can definitely say with our Hubble data is that we require both emission from an accretion disk and emission from a stellar population to explain the colors we see," said Farrell.

Such young clusters of stars are commonly found inside galaxies like the host galaxy, but not outside the flattened starry disk, as found with HLX-1. One possible scenario is that the HLX-1 black hole was the central black hole in a dwarf galaxy. The larger host galaxy may then have captured the dwarf. In this conjecture, most of the dwarf's stars would have been stripped away through the collision between the galaxies. At the same time, new, young stars would have formed in the encounter. The interaction that compressed the gas around the black hole would then have also triggered star formation.

Farrell theorizes that the possible star cluster may be less than 200 million years old. This means that the bulk of the stars formed following the dwarf's collision with the larger galaxy. The age of the stars tells how long ago the two galaxies crashed into each other.

Farrell proposed for more observations this year. The new findings are published in the February 15 issue of the Astrophysical Journal. Soria and his colleagues have published their alternative conclusions in the January 17 online issue of the Monthly Notices of the Royal Astronomical Society.

Black holes pervade the universe, celestial census indicates

After conducting a cosmic inventory of sorts to calculate and categorize stellar-remnant black holes, astronomers from the University of California, Irvine have concluded that there are probably tens of millions of the enigmatic, dark objects in the Milky Way -- far more than expected.

"We think we've shown that there are as many as 100 million black holes in our galaxy," said UCI chair and professor of physics & astronomy James Bullock, co-author of a research paper on the subject in the current issue of Monthly Notices of the Royal Astronomical Society.

UCI's celestial census began more than a year and a half ago, shortly after the news that the Laser Interferometer Gravitational-Wave Observatory, or LIGO, had detected ripples in the space-time continuum created by the distant collision of two black holes, each the size of 30 suns.

"Fundamentally, the detection of gravitational waves was a huge deal, as it was a confirmation of a key prediction of Einstein's general theory of relativity," Bullock said. "But then we looked closer at the astrophysics of the actual result, a merger of two 30-solar-mass black holes. That was simply astounding and had us asking, 'How common are black holes of this size, and how often do they merge?'"

He said that scientists assume most stellar-remnant black holes -- which result from the collapse of massive stars at the end of their lives -- will be about the same mass as our sun. To see evidence of two black holes of such epic proportions finally coming together in a cataclysmic collision had some astronomers scratching their heads.

UCI's work was a theoretical investigation into the "weirdness of the LIGO discovery," Bullock said. The research, led by doctoral candidate Oliver Elbert, was an attempt to interpret the gravitational wave detections through the lens of what is known about galaxy formation and to form a framework for understanding future occurrences.

"Based on what we know about star formation in galaxies of different types, we can infer when and how many black holes formed in each galaxy," Elbert said. "Big galaxies are home to older stars, and they host older black holes too."

According to co-author Manoj Kaplinghat, UCI professor of physics & astronomy, the number of black holes of a given mass per galaxy will depend on the size of the galaxy.

The reason is that larger galaxies have many metal-rich stars, and smaller dwarf galaxies are dominated by big stars of low metallicity. Stars that contain a lot of heavier elements, like our sun, shed a lot of that mass over their lives. When it comes time for one to end it all in a supernova, there isn't as much matter left to collapse in on itself, resulting in a lower-mass black hole. Big stars with low metal content don't shed as much of their mass over time, so when one of them dies, almost all of its mass will wind up in the black hole.

"We have a pretty good understanding of the overall population of stars in the universe and their mass distribution as they're born, so we can tell how many black holes should have formed with 100 solar masses versus 10 solar masses," Bullock said. "We were able to work out how many big black holes should exist, and it ended up being in the millions -- way more than I anticipated."

In addition, to shed light on subsequent phenomena, the UCI researchers sought to determine how often black holes occur in pairs, how often they merge, and how long it takes. They wondered whether the 30-solar-mass black holes detected by LIGO were born billions of years ago and took a long time to merge or came into being more recently (within the past 100 million years) and merged soon after.

"We show that only 0.1 to 1 percent of the black holes formed have to merge to explain what LIGO saw," Kaplinghat said. "Of course, the black holes have to get close enough to merge in a reasonable time, which is an open problem."

Elbert said he expects many more gravitation wave detections so that he and other astronomers can determine if black holes collide mostly in giant galaxies. That, he said, would tell them something important about the physics that drive them to coalesce.

According to Kaplinghat, they may not have to wait too long, relatively speaking. "If the current ideas about stellar evolution are right, then our calculations indicate that mergers of even 50-solar-mass black holes will be detected in a few years," he said.

Astronomers See Stars Slinging Comets at Earth for the First Time

Stars and comets make unlikely dance partners. Their gravitational partnership is one that astronomers have long suspected but have never seen — until now. For the first time, a Polish group has identified two nearby stars that seem to have plucked up their icy partners, swinging them into orbits around our sun.

The astronomers found the stellar duo after studying the movements of over 600 stars that came within 13 light-years of the sun. The new findings validate a theory born more than a half-century ago, and in doing so have also shown just how rare these stellar dances can be.

Out on the far edge of the solar system, hanging like wallflowers around the planetary dance floor, is the Oort Cloud. This icy group of objects were left over after the formation of the solar system, creating a giant shell enveloping our home system that extends from 66 times the distance to Neptune to 9.23 trillion miles (14.9 trillion kilometers) away from the sun. Astronomers think the Oort Cloud is a reservoir for long-period comets — those that take more than 200 years to orbit the sun. Comet Hale-Bopp, which has a 2,500-year orbit, is one of the most famous of these long-period comets.

Since the cloud's existence was first proposed by Jan Oort in the 1950s, astronomers have suspected that every so often, a passing star might be able to pick up an object and send it swinging on a wild ride through our solar system that ride would bring some of those comets streaming through the night sky for us to marvel at. Astronomers have spent years trying to find proof of these stellar dances, but none had been conclusively shown until now.

A new paper, accepted for publication in the journal Monthly Notices of the Royal Astronomical Society and published on the preprint database arXiv, describes how astronomers calculated the paths of nearly 650 stars, which they compared with the orbits of over 270 long-period comets. The study used a catalogue from the Gaia spacecraft, which has measurements for some 1.7 billion astronomical objects, along with surveys like Pan-STARRS, which looks for asteroids, comets and other small bodies in our solar system.

They created models for the star-comet pairs to rewind and replay their history. The astronomers then would "remove" a star from their model to see if that significantly changed the partner comet's orbit. If it did, the astronomers would know that the stars had tangoed with the comets.

"In our study, we discovered only two cases in which this actually happened, and yet, we observe dozens of comets every year," lead study author Rita Wysoczańska, an astronomer at the Institute Astronomical Observatory at Adam Mickiewicz University in Poland, told Live Science. "At this moment, we can say that the mechanism proposed by Oort is not sufficient enough to generate all comets we observe."

It's likely that the collective gravitational force of more distant stars can boost comets into long-period orbits. And once a comet enters the solar system, it can be further perturbed by the planets therein.

"I think, in general, it is hard to associate a particular comet with a particular star," said Coryn Bailer-Jones, an astronomer at the Max Planck Institute for Astronomy in Germany, who was not involved with the new study. "We must also consider the contribution of the galactic background potential, which is essentially the influence of all the other much more distant, but also much more numerous, stars in the galaxy."

Creating computer models to look at all of those influences, something called a multibody model, is a much more complex and computationally intensive task.

Additionally, information doesn't yet exist for every star. With current data, the astronomers had to rely on estimates for some of the stellar masses and movements. The astronomers hope that a future data release of the stellar survey they used can help shed more light on comet-star interactions.

Editor's note: This article was updated to indicate the astronomers looked at stars that came within 13 light-years of the sun, not 1.3 light-years as had been written.