# Leaving the Milky Way

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If we had a hypothetical spacecraft, would it be possible to exit our galaxy, wait for some time and enter back into another section? Will this method be more efficient that a more traditional flight path?

Not really, for the same reason that you cannot travel west by jumping up in the air and let Earth rotate underneath you, such that you land a little farther to the west.

The reason is that standing on Earth's surface, you already have a velocity toward the east which matches exactly the speed of the surface. Thus, in the reference frame of Earth, you simply jump up and down.

In the same manner, if you use your mad hypothetical spacecraft to fly "up", i.e. away from the Galactic plane, you already have a velocity of $sim$250 km s$^{-1}$ in the direction of the rotation of the plane, so that in the reference frame of the plane, you simply fly straight up.

Apart from this, in order to leave the Galactic plane, you need to fly roughly 500 lightyears. This will take a long time. You should stay home.

It depends greatly on the nature of your "hypothetical spacecraft".

Any spacecraft travelling through interstellar space will have to deal with interstellar gas and dust. At sufficiently high relativistic velocities, running into a hydrogen atom is like being hit by a high-energy cosmic ray, and running into a dust particle could seriously ruin your afternoon.

The intergalactic medium has substantially less matter than the interstellar medium within the galaxy. A route that exits the body of the galaxy by going "galactic north", then travels parallel to the disk, then re-enters the galaxy at another point will obviously be longer than a straight-line route, but it will encounter significantly less matter along the way. If the maximum safe velocity of the USS Hypothetical depends strongly on the density of the medium through which it's travelling, then the indirect route could be faster.

Whichever route you choose, this will take a long time. You should bring a book and plenty of snacks.

## The Orbits Of Ancient Stars Prompt A Rethink On The Milky Way's Evolution

Representation of the orbit of the star 232121.57-160505.4 in the Galactocentric cartesian frame, colour coded according to the time. The white dot represents the current position of the star. The black circled dot and the dashed circular line indicate the position and the approximated orbit of the Sun, respectively. CREDIT Cordoni et al

Theories on how the Milky Way formed are set to be rewritten following discoveries about the behaviour of some of its oldest stars.

An investigation into the orbits of the Galaxy's metal-poor stars - assumed to be among the most ancient in existence - has found that some of them travel in previously unpredicted patterns.

"Metal-poor stars - containing less than one-thousandth the amount of iron found in the Sun - are some of the rarest objects in the galaxy," said Professor Gary Da Costa from Australia's ARC Centre of Excellence in All Sky Astrophysics in 3 Dimensions (ASTRO 3D) and the Australian National University.

"We've studied 475 of them and found that about 11 per cent orbit in the almost flat plane that is the Milky Way's disc.

"They follow an almost circular path - very much like the Sun. That was unexpected, so astronomers are going to have to rethink some of our basic ideas."

Previous studies had shown that metal-poor stars were almost exclusively confined to the Galaxy's halo and bulge, but this study revealed a significant number orbiting the disk itself.

The Sun also orbits within the disk, which is why it manifests as the comparatively thin, ribbon-like structure easily visible from Earth in the night sky. In effect, we are seeing it edge-on.

"In the last year our view of the Milky Way has dramatically changed," said lead author Giacomo Cordoni from the University of Padova in Italy, who performed the bulk of the study while on a recent study placement at the ANU, funded by the European Research Council's GALFOR Project.

"This discovery is not consistent with the previous Galaxy formation scenario and adds a new piece to the puzzle that is the Milky Way. Their orbits are very much like that of the Sun, even though they contain just a tiny fraction of its iron. Understanding why they move in the way that they do will likely prompt a significant reassessment of how the Milky Way developed over many billions of years."

The ancient stars were identified using three very high-tech pieces of kit: ANU's SkyMapper and 2.3-metre telescopes, and the European Space Agency's Gaia satellite.

The low metal content was identified by the telescopes, and the satellite was then used to determine their orbits.

The results - crunched by researchers from Australia, Italy, Sweden, the United States and Germany - found that the orbits of ancient stars fell into a number of different patterns, all but one of which matched previous predictions and observations.

As expected, many of the stars had largely spherical orbits, clustering around the Galaxy's "stellar halo" - a structure thought to be at least 10 billion years old.

Others had uneven and "wobbly" paths assumed to be the result of two cataclysmic collisions with smaller galaxies that occurred in the distant past - creating structures known as the Gaia Sausage and the Gaia Sequoia.

Some stars were orbiting retrograde - effectively going the wrong way around the Galaxy - and a few, about five per cent, appeared to be in the process of leaving the Milky Way altogether.

And then there were the remaining 50 or so, with orbits that aligned with the Galaxy's disk.

"I think this work is full of important and new results, but if I had to choose one that would be the discovery of this population of extremely metal-poor disk stars," said Cordoni.

"Future scenarios for the formation of our Galaxy will have to account for this finding - which will change our ideas quite dramatically."

Cordoni's team included scientists from Italy's Centre of Studies and Activities for Space, the Max Planck Institutes for Astrophysics and Astronomy in Germany, the Massachusetts Institute of Technology in the US, Sweden's Uppsala and Stockholm universities, and Australia's Monash University, University of New South Wales and ANU.

The team included Australia's Brian Schmidt, winner of the 2011 Nobel Prize in Physics.

An advance version of the study is now available in the Monthly Notices of the Royal Astronomical Society.

## A Cosmic Mystery: Why is the Milky Way Galaxy Getting Gassier?

Astronomers have discovered a strange surplus of gas in the Milky Way galaxy.

Using 10 years of data from NASA's Hubble Space Telescope, the team of astronomers concluded that there is more gas coming into our galaxy than leaving it. Rather than an equilibrium of gas entering and escaping, there is a significant imbalance, though the team behind this finding has not yet found the source for this gaseous disparity.

The researchers used data from Hubble's Cosmic Origins Spectrograph (COS), which allows the space telescope to study objects that absorb or emit light and determine aspects such as their temperature, chemical makeup, velocity and density. With COS, the team could observe and track the movement of gases in the galaxy: the gases appear redder as they move away from our galaxy, and bluer as they get closer through the phenomena known as redshift and blueshift.

This allowed the researchers to see that there was more "blue" (entering) gas than "red" (exiting) gas. Although the researchers have not pinpointed the source of this imbalance, they think that it could possibly be caused by one of three things.

First, the astronomers think that this excess gas could be coming from the interstellar medium. Second, they suggest that the Milky Way is using its impressive gravitational pull to swipe gas from smaller, nearby galaxies, according to a statement.

Additionally, seeing as this study considered only cool gases, researchers think that hotter gases might also contribute to this finding.

Gases leave our galaxy when events like supernovae and stellar winds push them out of the Milky Way's galactic disk. When gases fall back into our galaxy, they contribute to the formation of new stars and planets. So the balance between the inflow and outflow of gases is important to regulating how objects such as stars form in galaxies like ours.

"Studying our own galaxy in detail provides the basis for understanding galaxies across the universe, and we have realized that our galaxy is more complicated than we imagined," co-author Philipp Richter of the University of Potsdam in Germany said in the statement.

This research will be published in a study in The Astrophysical Journal.

## Astronomers detect star leaving the Milky Way at record speeds

An international team led by astronomers from Queen's University Belfast has identified the fastest ever star on an escape trajectory from the Milky Way – the white dwarf US708, which is traveling at a staggering 1,200 km per sec (746 miles per sec). The discovery of this star may shed light on the astronomical events that are vital to the calculation of distances in our universe.

The team used data gathered by the Pan-STARRS1 telescope located on Mount Haleakala, Maui, to determine the runaway star's speed and direction. From this it was concluded that US708 originally belonged to a binary star system, whence it was paired with another enormous white dwarf.

It is believed that binary systems of this kind often result in a thermonuclear explosion known as a 'Type la' supernova. This particular form of star death occurs when the incredibly dense larger white dwarf feeds on the stellar material of its partner (in this case the smaller white dwarf US708) until it reaches critical mass – the equivalent of 1.4 solar masses. This is known as the Chandrasekhar limit, and very soon after the star explodes in spectacular thermonuclear fashion.

It is hoped that further examination of US708 may shed light on the phenomenon that shunted the star into its escape trajectory, as this Type la supernova is a vital astronomical marker used by scientists to determine the distances of far off galaxies.

Astronomers are able to use these distinctive supernovae as distance markers because they always throw off an identical amount of light. Scientists can then observe dimming in the known luminosity of this brilliant light source, and apply the inverse square law in order to ascertain how far away the supernova is, and also the galaxy to which it belongs.

"Several types of stars have been suspected of causing the explosion of a white dwarf as supernova of type Ia. Until now, none of them could be confirmed," says Stephan Geier, fellow of the European Southern Observatory and leader of the study. "Now we have found a delinquent on the run bearing traces from the crime scene."

## Leaving the Milky Way - Astronomy

What will eventually happen to the interplanetary probes Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2, which are now on an escape trajectory from our solar system? Will they ever escape the Milky Way?

The probes will continue out of our solar system and will travel through space in the same direction in which they left the solar system. Pioneer 10 is heading toward Aldebaran and will get there in approximately two million years. Pioneer 11 is heading toward the constellation Aquila and will reach a star in four million years.

It is doubtful that the spacecraft will ever be able to leave the Milky Way, as they would have to attain a velocity of 1000 kilometers/second, and unless they get a huge, huge, huge velocity boost from something unexpected, they will probably end up being in the Milky Way's rotation forever. However, even if they could leave the Milky Way, it wouldn't be for millions and millions and millions of years, and we would long ago have lost communication with the Voyagers, with which we will theoretically be in contact until 2020.

## Leaving the Milky Way - Astronomy

Телескоп/объектив съёмки: Sigma 20mm Art f/1.4

Камеры для съемки: Canon Ra

Программы: Adobe Photoshop CC 2019 · Starry Landscape Stacker · Adobe Lightroom CC

Кадры: 5x13" (1' 5")

Накопление: 1' 5"

Сред. возраст Луны: 23.78 дней

Средн. фаза Луны: 32.97%

Astrometry.net job: 4584670

Разрешение: 1900x1311

Местоположения: Deerlick Astronomy Village, Crawfordville, GA, Соединенные Штаты Америки

Источник данных: Путешественник

#### Описание

The Milky Way rising over Deerlick Astronomy Village in May 2021

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• Команда локализации: José J. Chambó · Jean-Charles Schmidt · Laurent Schmitz · Lauri Kangas · Samuli Vuorinen · Jussi Kantola · Koen Dierckens · Hans Lambermont · Michael Wilk · Marc Demierre · Rinke Kallenkoot · Leon Handreke · Sedat Bilgebay · Onur Atilgan · Güven Özkan · Ahmet Kale · Ardian Xherrahi · Milosz Wasilewski · Glauco Hass · Konstantinos Stavropoulos · Pavel Syrin · Ken-ichiro Tanaka · Łukasz Sujka · Jean-Baptiste Auroux · Nadir Teymurov · Hartmuth Kintzel · 雅痞张
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## Imaging the Milky Way

There is a fundamental problem when taking images of the Milky Way, that of light pollution, formally called skyglow. In other articles in the digest, the method of removing light pollution from images is described, but this will not work when there are large areas of nebulosity in the image, as there will be, of course, when imaging the Milky Way. It would be difficult for any procedure to distinguish between skyglow and faint nebulosity. The obvious solution is to image from a very dark sky location where there is no skyglow as was done for the second and third images described below. However, if the region of the Milky Way to be imaged is overhead and the skyglow is not too prominent (meaning that one could see the Milky Way above) it is possible to remove it as in the first example.

Imaging the Cygnus region of the Milky Way

A total of 35, 13 second, frames were taken of this region from a relatively dark sky location in Cheshire where the Milky Way could just be discerned overhead. A Sony A7S was used at an ISO of 800 with a (very sharp) Zeiss 45mm, f/2, Planar lens stopped down to f/4. Both Jpeg and raw frames were captured. As I have sometimes found, a better result was obtained from processing the Jpeg frames which brought out the H-alpha emission better. The frames were stacked using Sequator and the resulting 16-bit Tiff file imported into Adobe Photoshop for processing (Affinity Photo could have been used equally well). The image below shows the output image produced by Sequator having stretched it somewhat to bring out the skyglow.

Removing the skyglow

This was only possible as the Cygnus region was virtually overhead. In this case, it is a reasonable assumption to assume that it will be constant across the image and so a sample of the image at its darkest point away from any nebulosity could reasonably be used to represent the skyglow in the image. Pleasingly, Cygnus includes the dark band of dust that obscures the Milky Way in the Cygnus Rift. The image was duplicated and a Gaussian Blur of 10 pixels applied to average out the sky background which was then sampled with the brush tool in the darkest region of the image that could be seen. This colour was then used to completely paint over the upper layer hoping that it would represent the skyglow across the whole of the image.

The two layers were then flattened using the ‘Difference’ blending mode to give an image showing just the brighter stars and no evidence of the Milky Way.

Stretching the image

The image was stretched using several applications of the curves function as shown in the figure below .

The stretched result showed the Milky Way well.

A method of increasing the local contrast to better delineate the Milky Way is to use the ‘Unsharp Mask’ filter with a very large radius but small amount (the opposite of its use to sharpen an image).

In this image, applying local contrast to the stretched image including the stars gave a good result as shown below along with an annotated version.

[A sometimes better approach is to separate out the ‘stars’ and the background Milky Way, enhance the Milky Way applying some local contrast and then ‘put’ the stars back again. One advantage is that, using the opacity slider when using the ‘Screen’ blending mode as the Milky Way and stars layers are flattened one can control how prominent one wishes the stars to be. To separate out the stars and Milky Way, the ‘Dust and Scratches’ filter is applied to the stretched image with a radius of

15 pixels. The stars will disappear leaving just the background Milky Way image. This is saved as ‘Milky Way’. The original image is brought back and copied and pasted over the Milky Way image to give two layers which are flattened using the ‘Difference’ blending mode. This leaves the ‘Stars’ image that can be saved or left in the workspace. Local contrast is applied to the Milky Way image and the Stars image copied and pasted over it. The stars are then added into the image using the ‘Screen’ (or ‘Lighten’ – try both) blending mode when flattening the two layers using the opacity slider if desired.]

I was impressed, and somewhat surprised, as to how well the H-alpha emission had been captured despite the fact that the IR cut-off filter has not been modified to allow more of its light to fall on the sensor. I know that the H-alpha emission shown is real as I have imaged the region including Deneb and Sadir using a monochrome cooled CCD camera and H-alpha filter to overlay an H-alpha layer onto a RGB image.

Cygnus region with an H-alpha overlay

#### The Heart of the Milky Way imaged from New Zealand

This first image of the central part of the Milky Way was taken from a dark sky location in New Zealand using my first lightweight imaging system. This used a Panasonic GX1 Micro 4/3 camera and Panasonic 20 mm, f/1.7, lens stopped down to f/4 (giving an effective 35mm focal length of 40 mm) mounted on a Nanotracker. It was not necessary to remove any skyglow and the image was simply stretched a little and its local contrast increased as described above. An annotated version is added.

The Central Region of the Milky Way imaged from New Zealand

On a second trip to New Zealand, I used a Sony A5000 camera with, for this image, a Samyang 12 mm, f/2, lens stopped down to f/4.

A crop of the central region

#### The Southern Cross Region of the Milky Way imaged from Lake Tekapo in the Aoraki Mackenzie Dark Sky Reserve

This is a ‘gold tier’ dark sky site which has one of the darkest skies in the world and home to the Mt John Observatory. The same imaging setup was employed but, this time, the Sony A5000 APS-C camera was coupled with the Zeiss lens 45mm, f/2, as used for the Cygnus region image. A total of 4 panes were taken each derived from 20, 15 second, exposures captured at an ISO of 800 in both raw and Jpeg. The raw files were converted into Tiff files using Raw Therapee and stacked in Sequator. These overlapping panes were then composited into one image using Microsoft ICE. A little stretching and local contrast enhancement was done as described above to give the image below.

Composite image made from 4 panes

## Dark matter could be powering a galaxy that orbits the Milky Way until they collide

Have you ever looked up into the darkness and wondered what is on the edge of the Milky Way? Stars, an orbiting galaxy, and an almost unfathomable amount of dark matter.

Astronomers who were virtually navigating through the furthest reaches of our galaxy through data from the NASA NEOWISE and ESA Gaia missions have come up with a new map of what is floating out there. In what is known as the galactic halo, something appears to be pulling at the Large Magellanic Cloud (LMC), a much smaller galaxy that is still only on its first orbit around the Milky Way. That something is also leaving a wake of stars trailing behind it as the LMC sails on. The unseen forces that are probably doing this come from the Dark Side.

#### More NASA

Darth Vader may have not had much use for dark matter, but Nicolas Garvito-Camargo of the University of Arizona, who recently led the study, coauthor Rohan Naidu of Harvard, and their colleagues believe it could be the reason that the LMC will eventually crash into the Milky Way—something the Death Star never got to do.

“Stars in the outer regions of the galaxy were observed moving faster than expected given the amount of mass observed in the stars,’ Garvito-Camargo told SYFY WIRE. “This was one of the first pieces of evidence that there should be more mass in the galaxy than what is observed.”

Because there is more mass in our galaxy than they could see, the astronomers came to the conclusion that the rest of the mass must be coming from the invisible stuff that is dark matter. What makes dark matter truly invisible (instead of just hidden) is that it emits no light and cannot be illuminated. This kind of matter was first hypothesized about when the Vera Rubin Observatory found a disconnect between the mass of the stars in the galactic halo and the speed at which they were moving. When it measured the rotation curve of neighboring galaxy Andromeda, that was one of the first instances in which scientists suspected dark matter.

Andromeda’s strange rotation curve meant the masses and speeds of orbiting stars in the galaxy, compared to their radial distance from its center, did not add up. The rotation curve of the Milky Way turned out to have the same problem. Measuring its rotation curve and quantifying the invisible matter gave an idea of how much dark matter they were dealing with in the galactic halo. Stars also helped light the way to what could not be seen. Garvito-Camargo and his team used NEOWISE data to measure starlight in a specific infrared wavelength that gave away the most massive stars in the halo.

“With this information, we could distinguish which stars were the giants, and once we selected them, we could calculate what would be the distance to those stars to see the apparent brightness we were measuring, not only in the infrared but also in those from Gaia,” he said. “We could do this since we have models that tell us what the brightness of those stars is at a given distance.”

By mapping the galactic halo, the scientists were also given the chance to test out the properties of its dark matter by using the Cold Dark Matter Theory. Assuming the dark matter is cold means that it moves slowly as opposed to being actually frozen. Because galaxies and galaxy clusters have vast quantities of this slow, dense dark matter, its gravity holds them together and keeps stars, planets and other objects from fling everywhere in space. It would probably be impossible to send spacecraft anywhere if it weren’t for the gravitational grip of dark matter keeping things where they should be.

“Using another dark matter theory would have an impact on the morphology of the wake,” said Garvito-Camargo. “We can make the analogy to viscosity. More viscous fluid would behave differently even if it is exposed to the same perturbations. Similarly, some dark matter models would react differently to the LMC. As a result, the morphology of the wake would be different.”

Because the dark matter in the galactic halo generates such an immense amount of gravity, it will keep pulling the Large Magellanic cloud closer and closer to the Milky Way with each subsequent orbit until they merge in another 2 million years or so.

## Leaving the Milky Way - Astronomy

The Milky Way's cosmic clouds of stars and dust stretch across this picture taken May 2001 from Hawaii. In the foreground is an "ahu hoku" - a star marker or star altar - built up of rocks topped with a white piece of coral glowing in the moonlight.

The word galaxy comes from a Greek word meaning "milky circle" or, more familiarly, "milky way." The white band of light across the night sky that we call the Milky Way was observed and described poetically long before Galileo examined it with a small telescope. What he discovered was a multitude of individual stars, "so numerous as almost to surpass belief."

Today we know that the Milky Way is our home galaxy - a vast rotating spiral of gas, dust, and hundreds of billions of stars. The Sun and its planetary system formed in the outer reaches of the Milky Way about 4.5 billion years ago.

In the center of the galaxy is the bar-shaped galactic bulge which harbors a supermassive black hole with a mass equal to that of about 3 million suns. Surrounding the central bulge is a relatively thin disk of stars about two thousand light years thick and roughly 100,000 light years across. Almost all the stars seen by the human eye are in the thin disk, which accounts for about 90% of the visible light in the Milky Way.

Giant clouds of dust and gas in the disk and bulge absorb light from the stars and give the galaxy its patchy appearance. These clouds are the stages on which the long-running drama of stellar evolution is played. Dust and gas collapse to form stars, then nuclear fusion reactions in their interiors build up heavier elements and release energy which is radiated as starlight.

As the stars' nuclear energy supply is exhausted, most stars will expand to become red giants, then shrink to a small dense state called a white dwarf star. Massive stars will explode as supernovas, leaving behind neutron stars or black holes. The explosions disperse heavy elements manufactured by the stars, thereby enriching the galaxy with elements necessary to form planets.
More on Stellar Evolution

The Galaxy's bright stellar disk is embedded in a faint, thicker disk of old stars. This disk, which has a thickness about 3 times that of the thin disk, may have been the original structure from which the thin disk condensed, or it could have been thickened by a collision with a smaller galaxy ten billion years ago.

Schematic of Milky Way showing the dark matter halo (gray), globular clusters (red circles), the thick disk (purple), the stellar disk (white), the stellar bulge (red-orange), and the central black hole (black dot). The stellar disk is about 100,000 light years in diameter. The dark halo extends to a diameter of at least 600,000 light years. (Illustration: CXC/M.Weiss)

Surrounding the thick Galactic disk is an extremely faint halo that contains the oldest stars in the Galaxy. These stars are located in globular clusters, dense swarms of about 100,000 stars. The Galactic halo is dominated by dark matter, a still mysterious form of matter that cannot be seen with any type of telescope, but is detected by its gravitational effects. Studies of the motions of stars and gas in the Milky Way indicate that the mass of the dark matter halo is about twenty times greater than the mass of all the stars in the galaxy.
More on Dark Matter

It is thought that the various components of our Galaxy were put together about 12 billion years ago through a succession of mergers that are continuing even today. Clouds of gas are observed to be falling into our galaxy, and recent evidence indicates that a small galaxy on the far side of the Milky Way is being torn apart and assimilated into the Galaxy.

These processes emphasize that the Milky Way is not an island universe, but a member of a small cluster of galaxies called the Local Group. The Local Group contains about 3 dozen known galaxies, clumped in two subgroups around two massive spiral galaxies --the Milky Way, and the Andromeda Galaxy. In several billion years it is possible that the Milky Way and Andromeda will collide and merge to form one huge elliptical galaxy, so enjoy the Milky Way while you can!

This artist's animation takes a virtual voyage from Earth through the Milky Way galaxy to the outer reaches of the Local Group of galaxies. Leaving Earth we pass the planets Venus and Mercury, then cruise by the Sun, the star of our solar system. We then travel about 24 trillion miles, or 4 light years, before we pass our neighboring stars in the Alpha Centauri complex.

At a distance of a few hundred light years we encounter clouds of dust and gas illuminated by brilliant clusters of young stars. These clouds and star clusters are part of the Orion spiral arm. As we move further out, fifty thousand light years from the Sun, other spiral arms of the Galaxy come into view along with the central bulge, where the Galaxy's supermassive black hole is located. Finally, from a distance of a few million light years, we see the Galaxy as part of the Local Group.

From the tranquil, wide-open spaces between the galaxies of the Local Group, we now zoom into where the action is - the brightly lit, crowded center of the Galaxy, the "Broadway" of our sprawling stellar metropolis. This 400 by 900 light-year mosaic of several Chandra images of the central region of our Milky Way galaxy reveals hundreds of white dwarf stars, neutron stars, and black holes bathed in an incandescent fog of multimillion-degree gas. The supermassive black hole at the center of the Galaxy is located inside the bright white patch in the center of the image. The colors indicate X-ray energy bands - red (low), green (medium), and blue (high).

The Chandra mosaic gives a new perspective on how the turbulent Galactic Center region affects the evolution of the Galaxy as a whole. Large quantities of multimillion degree gas appear to be escaping from the center into the rest of the Galaxy. The outflow of gas, chemically enriched from the frequent destruction of stars, will distribute these elements into the galactic suburbs.

A composite of images made at X-ray (blue), infrared (green), and radio (red) shows the relation between hot gas (X-ray), cool gas and dust (infrared) and high energy electrons trapped in the magnetic field in the Galactic center (radio). Because it is only about 26,000 light years from Earth, the center of our Galaxy provides an excellent laboratory to learn about the cores of other galaxies.

Select the wavelengths you would like to view below:
X-ray (blue), Infrared (green), Radio (red) or all three.

X-ray (blue), Infrared (green) and Radio (red) Composite.

## Leaving the Milky Way - Astronomy

Do you have in mind a photo of the Milky Way, but don’t know when it occurs? Good news, you're at the right place!

This tutorial will help you learn how to use PhotoPills’ Night Augmented Reality to figure out the exact date and time the Galactic Center of the Milky Way will be exactly where you want.

On the other hand, if you prefer planning the Milky Way on a map, please, have a look at the following tutorial: How To Plan The Milky Way Using The 2D Map-Centric Planner.

Milky Way: The Definitive Photography Guide

You can also take a look at the following video. We show you how to plan the Milky Way using both: the Night Augmented Reality view and the 2D Map-Centric Planner.

Finally, make sure you don't miss the article “How To Shoot Truly Contagious Milky Way Pictures”. You'll learn how to turn your Milky Way ideas into real images, step by step from inspiring sources and equipment to camera settings.

### 1 Two crucial Milky Way facts you should know

Let’s say that during one of your scouting sessions you come across this isolated rock. It’s located in a powerful landscape with no light pollution (complete darkness). It has the perfect conditions for a stunning Milky Way shot.

Find a powerful location and let your imagination fly.

The Milky Way moves in the sky following Earth’s rotation as the stars move, this means you will have different compositions at different times of the night. You can get the band of the Milky Way in vertical, diagonal or horizontal orientation.

But, before you start brainstorming like crazy, there are two CAPITAL facts about the core of the Milky Way you should keep in mind:

#### You’ll find the core in the southern skies

Knowing the direction where it is possible to find the core of the Milky Way is mandatory.

Don’t waste your time designing images that are not possible. These are the general rules depending on the Hemisphere you are:

• Northern Hemisphere: look towards the southern skies to see the galactic core. The core will start to be visible due southeast (Spring), due south (Summer), or southwest (Fall).
• Southern Hemisphere: also look towards the southern skies to see the galactic core. In this case, the core will start to be visible due southwest (Spring) or southeast (Fall and Winter).

In conclusion, don’t look for the core of the Milky Way in northern directions. When brainstorming, think about different compositions with the galactic center in the southeast, south or southwest.

#### Same location, same direction, same altitude

“For a given location and direction (azimuth), the galactic center will ALWAYS be at the same altitude in the sky.”

This means that if you go to the same location in two different dates, look towards the same direction and wait until the galactic center is in that direction, you'll see it at the same altitude in the sky.

No matter the date, for a given location, when the galactic center is in one direction, it always has the same altitude.

Thus, given a location, the galactic center always rises in the same direction. Also, it always sets in the same direction.

The practical application of this fact is clear: for example, once you know the azimuth in which the galactic center rises, just choose the shooting spot in a way that the azimuth of the galactic center is just where you want relative to the main subject of your photo (rock, tree, lighthouse, building, etc).

In other words, when you find a location you like, proceed as follows:

• Decide the position of the galactic center in the sky. Most times your initial shooting spot will not be right. You'll have to move.
• Use PhotoPills‘ Night AR to find out the azimuth in which the galactic center is at the desired altitude and orientation.
• Again, use the Night AR tool to choose the shooting spot that gives you the composition you want.

### 2 When does Milky Way hunting season start?

In other words, when should you start looking for the core of the Milky Way? When will it be visible? Or even better, when is the best time of the year to shoot the Milky Way?

During part of the year, the core of the Milky Way is not visible because it is blocked by the sun. It's when the galactic center is only above the horizon during daylight hours.

When planning the Milky Way, you are only interested in looking into the period of the year the galactic center is visible during nighttime. Thus, knowing the starting and ending dates of the best period of the year to shoot the Milky Way is important to narrow the search and get results faster.

#### Northern Hemisphere

In the Northern Hemisphere, the core is visible from March to October. But the best time for viewing it is from late April to late July, because the galactic center is visible for longer during the night. Don’t look for it from November to February.

The Galactic Center is visible from March to October and not visible from November to February.

In late February, the core becomes visible in the pre-dawn hours just before sunrise, and remains above the horizon during daylight hours. As months go by, the core becomes visible for longer and longer each night, being June and July the months with longer visibility. During this time of year, the core will be visible all night.

From July on, core visibility begins to decrease and best viewing time moves towards after dusk, until it becomes totally invisible again in winter.

In conclusion, if you live in the Northern Hemisphere, late April is a good moment to start planning the Milky Way, being June and July the best months.

#### Southern Hemisphere

In the Southern Hemisphere, the core is visible from February to October, being in the middle of the winter, June and July, when the core is most visible. Again, don’t look for it from November to January.

The Galactic Center is visible from February to October and not visible from November to January.

People living in the southern hemisphere enjoy visibility longer because the peak occurs in winter, when days are shorter and nights are longer.

If you live in the Southern Hemisphere, mid-April is a good moment to start planning the Milky Way.

#### Consider Moonphase

Most times, you’ll want to have complete darkness when shooting the Milky Way. Therefore, when planning, you need to take into account the phase of the moon. You need to have no moon!

As a result, you’ll plan Milky Way shots happening during new moon and the 4 days before and after it.

• Don’t forget that in the winter (Northern Hemisphere) and summer (Southern Hemisphere) you can still see the Milky Way, just not the core.

### 3 Decide where you want the core of the Milky Way

Let’s go back to the example. Remember that you’re just in front of the rock. Now, place yourself in a spot leaving the southern sky behind the rock and start brainstorming. This is your initial choice as shooting spot.

Stay on this initial shooting spot and start planning.

Imagine, that during the creative process, you realize you love the position of the rock in relation to the sky. The top is pointing towards the stars in a diagonal orientation.

What if you could shoot the core of the Milky Way following the same diagonal?

A representation of the target photo.

You see the horizon behind the rock. Therefore, you need to find a shooting spot from where it's possible to see the galactic center rising near the rock, just on the left-hand side.

Let’s use PhotoPills’ Planner and Night AR tool to plan the shoot!

Let’s find when it happens!

### 4 On the Planner, place the Observer's pin on the initial shooting spot

You have everything you need to start planning the shoot:

• Initial shooting spot.
• Desired position of the core of the Milky Way.
• Planning period: from April new moon to August New moon.

On the Planner, place the Observer’s pin (red pin) on the initial shooting spot. AS you are in situ, right on this spot, tap on the GPS button you find on the map.

You’ll see how the Observer’s pin is automatically placed right where you are. If necessary, drag and drop it to re-adjust its position.

Notice that I've drawn on the screenshot a small black X that shows the position of the rock on the map.

Map view of the location in study. The black X shows the position of the rock.
Observer’s pin placed on the initial shooting spot, near the rock.

### 5 Set the date to the next new moon happening between April and August

Imagine it’s February 12th 2014 and you are planning this shoot. You've learnt in Step 2 that the Milky Way hunting season starts in April. Then, you need to set the date of the Planner to April new moon.

The easiest way to do it, it’s by tapping on the Moon picture you see on the rise and set information panel you find just above the map.

The picture of the moon shows how you'd see the moon on the selected date and time if you were on the location of the Observer's pin.

Tap once on the picture, you’ll see that time jumps forward to the date of the next main moonphase. In this case, you’ve landed on February 15th 2014, Full Moon.

Tap once on the picture of the moon to jump forward in time to the next main moonphase: February 15th, Ful Moon (100%)

Keep tapping on the picture of the moon until you set the date to April new moon, which occurs on the 29 th .

• Double tap on the picture of the moon to jump backwards in time to the main past moon phase (new, last quarter, full moon, first quarter).

### 6 Use the time bar to set the time to the end of the evening astronomical twilight

You want complete darkness. Set the time to the end of the evening astronomical twilight, when night begins.

Check on the twilight information top panel when the evening astronomical twilight ends: 10:20pm.

Now, drag the time bar towards the left to go forward in time until the time is set to 10:20pm.

The twilight information top panel tells you that the evening astronomical twilight ends at 10:20pm.
Time set to 10:20pm, when the evening astronomical twilight ends.
• You can set date and time numerically by tapping once on the centre of the time bar.
• To do it faster, tap on the right hand side of the time bar to jump to the next important event: golden hour begins, sunset, blue hour begins, civil twilight ends, nautical twilight ends, astronomical twilight ends, moonset, sunrise, moonrise…. Keep tapping on it until you set the time to the end of the evening astronomical twilight.
• On the other hand, tap on the left hand side of the time bar to jump backwards in time to the previous important event.

### 7 Tap on the Night AR button, check the Milky Way and adjust your position

You’ve just set the initial shooting spot, date (April new moon) and time (the end of the evening astronomical twilight).

Let’s have a look at the Milky Way. Tap on the Night AR button you see at the bottom of the screen and wait a few seconds until the augmented reality is stable. Shaking the device helps! Make sure you are away from any electronic device or magnetic field because they may interfere with the sensors of the device.

Now, you can preview the position of the Milky Way for the selected Observer's pin location, date and time.

Notice that the galactic center is not aligned with the rock. Use the AR view to explore the sky until you find the galactic center, it's the brightest part.

Night AR view of the rock from the initial shooting spot on April 29th at 10:20pm, at the end of evening astronomical twilight. The Milky Way is not where you want.
Position of the core of the Milky Way seen from the initial shooting spot at the end of evening astronomical twilight.

You want to shoot the core aligned with the rock just above the horizon level. Thus, you need the galactic center to rise near the rock, just on the left-hand side.

Swipe your fingertip on the AR view, from right to left, to move time forward until the core of the Milky Way rises and check the time that’s written on the top left-hand side corner of the screen: 12:49am.

Change time continuously, keep swiping your fingertip on the screen and observe how the core moves in the sky. Do it until you see the core next to the rock. This happens at 6:48am.

The Milky Way is far to much vertical, you want it in a diagonal orientation. Furthermore, 6:48 am is daytime, and you are only interested in results happening at night (complete darkness). In conclusion: you need to change the shooting spot.

Night AR view of the rise of the galactic center seen from the initial shooting spot around 12:49am and Antoni Cladera in a sport style.
• Notice that the background color of the two AR screenshots is different. The first one is clear (no color), however the second one has a darker color.
• The darker shade indicates that the result doesn’t occur at night and, therefore, we are not interested in it.
• We implemented the color change to help you disregard the results that occur during twilights, golden hour and daytime hours.

Move time backwards until you get the Milky Way in a more diagonal orientation, similar to what you want. To do it, just swipe your fingertip on the AR view, from left to right. This happens at 3:41am.

Now, use the Night AR view to guide yourself and move around until you find a spot from where you see the galactic center near the rock, just on the left-hand side.

Night AR view of the core of the Milky Way, seen from the initial shooting spot, with a diagonal orientation.
Move around until you find a new shooting spot from where you can take the photo you want.

What you see now is pretty close to what you want. Shut the AR view, go back to the Planner and place the Observer's pin on the new shooting spot.

Again, make sure that the Milky Way is right where you want. If needed, change time to re-adjust the position of the Milky Way.

That's it! You've just planned it!

Now you know that you must be on the final shooting spot ready to press the shutter on April 30th 2014 at 3:41am.

Night AR view when the core of the Milky Way is placed right where you want, seen from the final shooting spot.

Keep in mind that the AR view is showing you what you'd see if you were right on the location of the Observer's pin. If you change your position, you have to re-adjust the position of the Observer's pin to make sure you see the right representation of the Milky Way.

If the shooting time is too early in the morning for you, check the next new moon:

• Tap on the moon picture until you set the date to the next new moon: May 28th 2014.
• Set the time to the end of the evening astronomical twilight.
• Change the time until the core of the Milky Way is placed where you want.

The new shooting date and time: May 29th 2014 at 2:27am.

Tap on the moon picture until you set the date to the next new moon: May 28th 2014.

Keep repeating this process until you find a date and time that suits your schedule.

It’s also a good idea to check the 4 days before and after the new moon. If you do a single tap on the right-hand side of the Nigh AR screen, time will jump forward 24h. Do a single tap on the left-hand side of the screen to jump to the previous day.

I invite you to play a little bit with PhotoPills' Night AR tool and why not try to imagine, plan and shoot crazy images like the ones we took in our Star Wars Tribute.

Please, if you have questions, don't hesitate to use the comment section below.

## Keeping Time in the Milky Way with Chemical Clocks

Stellar age is an extremely valuable parameter to constrain because it introduces time into our study of astronomical objects. Pairing the observed properties of stars with time opens up a rich new dimension in the study of our Galaxy and beyond. For example, when we pair stellar age with stellar kinematics, we can dynamically trace stars back to their birth locations to study things like Galactic evolution and star formation in detail. When we consider stellar age in our study of exoplanets, we can peer into the planet formation and evolution process . When we pair stellar age with stellar chemical abundances , we can trace the evolution of specific elements over time in the Galaxy. Weaving time into these various analyses opens up a new realm of insight that enhances our understanding of the Universe. However, with this all said, stellar age is extremely difficult to constrain.

### Stellar Ages are Hard to Determine

Some methods of constraining stellar ages include using photometry, dynamics, gyrochronology , and the abundances of individual elements like lithium in stars. For example, the locations of stars on the color-magnitude diagram (CMD), which are determined by photometry , can hint at stellar age. Many stellar and Galactic astronomers fit isochrones, lines of constant age in the CMD, to the photometric data of a single or group of stars to estimate their age. However, this method relies on very well-constrained dust parameters between the observer and the object. Gyrochronology, using stellar rotation to estimate age, is another effective method, but it requires knowledge of the inclination of the star, something that is often difficult to determine. We can also use lithium abundances to estimate stellar age. Lithium, however, is only an effective age indicator in young stars with convective envelopes. As you can probably tell, there are tons of ways to estimate stellar age, but they all suffer from various limitations and uncertainties.

### Abundance Ratios of Certain Elements Track with Age

An interesting, and somewhat new, avenue for probing stellar age is through the use of chemical clocks. Chemical clocks are sets of elemental abundance ratios that have been observed to track with stellar age. The idea behind chemical clocks is rooted in the notion that different families of elements are expelled into the interstellar medium (ISM) on different time scales (see Figure 1). For example, elements like Mg, Al, and Ti that are produced in dying massive stars, which live short lives that end in core-collapse supernovae , follow much different timescales than elements like Ba and Y produced primarily in low-mass stars, which have much longer lifetimes and subsequently take longer to spread their nucleosynthetic products out into the ISM. This means that the ratios of various abundances in the ISM are constantly changing. When a star is born, it traps with it the chemical abundances of the ISM at the time of its birth like a time capsule and carries them with it throughout most of its life. Thus, the ratios of certain elements in a star could probe at what point in the Milky Way’s chemical evolution (and thus in time) the star was born.

Figure 1: A cartoon depicting the different timescales of chemical enrichment from various sources, the concept behind chemical clocks. Core-collapse supernovae, which come from short-lived massive stars, for example, dominate the chemical enrichment of the Milky Way early on. AGB stars, which originate from long-lived low- and intermediate-mass stars, start contributing to Galactic chemical enrichment later on. Figure 1 in Jacobson & Frebel (2014)

### Testing Chemical Clocks in Wide Binaries

The authors of today’s paper set out to investigate just how reliable chemical clocks are at keeping time by testing their consistency in wide binaries. Wide binaries are pairs of stars that were born together and orbit a common center of gravity. As their name implies, wide binaries have large separations, making them easier to study observationally. Wide binaries are a great way to test chemical clocks because they consist of two stars that share an age. Today’s authors investigate various chemical clock abundance ratios in 36 pairs of wide binaries to see which chemical clocks are most consistent among stars born at the same time.

The authors are first able to recreate the result found in previous studies that wide binaries are more chemically similar in their elemental makeup than random pairs of stars in the field. This makes sense. Stars born in the same place should share the same chemical composition because the interstellar medium is understood to be very homogeneous on small spatial scales. The chemical abundances of stars directly reflect the chemical abundances of the material from which they were born, so if the interstellar medium is well-mixed, and stars share a birth place and age, then they should share a similar chemical profile.

Figure 2: The consistency in the abundance of various chemical clocks between both components of wide binaries. The x-axis in each subplot is the abundance in the indicated chemical clock for one component of the binary (A), and the y-axis is the same for the other component (B). The tighter the 1 to 1 relationship in a subpanel, the more consistent a chemical clock between stars in the binary pair. [Sc/Ba], [Al/Ba], and [Ti/Ba] (all in the 4th row), among others, stand out as chemical clocks that appear to be promising age indicators. Figure 8 in the paper.

The authors then make an interesting discovery: when they investigate chemical clocks among wide binaries, they find that components of wide binaries tend to be even more similar in chemical clock abundances than other elemental abundances, as seen in Figure 2. They find that even when components of a wide binary are quite dissimilar chemically in [X/Fe], as is the case in one particular pair in their sample (black box in Figure 2), they are still very consistent in chemical clock abundances. This result suggests that chemical clocks could be effective age indicators even when stars are extremely dissimilar in other elements. The authors highlight that three chemical clocks in particular, [Sc/Ba], [Al/Ba], and [Ti/Ba], seem to be the most consistent among wide binaries and thus the most promising indicators of age.

What is next for the field of chemical clocks? One new avenue involves calibrating chemical clocks using stars with ages derived through other means, such as gyrochronology. This way, we can create an empirical, observed relationship between a star’s abundance in a chemical clock and its age. These empirical relationships will likely vary with Milky Way location, but they will open up a new avenue of probing stellar age in stars with a variety of parameters. With chemical clocks, we can hopefully expand our stellar age toolbox and allow for more checks on stellar age, an important parameter in observational astronomy.