Posts tagged andromeda
Posts tagged andromeda
Structure and Evolution of Galaxies
As much as we know about galaxies, the evolution of galaxies is one of the mysteries of the Universe. By looking far out into space and equivalently, far back into time, astronomers can look at the oldest galaxies and make comparisons with modern-day galaxies to get a sense of how they’re evolving. To really understand galaxy evolution, though, one must understand the details of the structure of galaxies; therein lies many wondrous secrets of galaxies. We will focus on spiral galaxies:
The Structure of Spiral Galaxies
Most spiral galaxies have similar features, the most significant of which include a dark matter halo, the galactic disc, spiral arms, a central bulge (most containing a bar-like structure), and a stellar halo.
The dark matter halo is believed to be the main structural component of the Milky Way galaxy whose functional purpose is primarily to hold together the galaxy; without it, the galaxy would not be gravitationally bound. The dark matter halo is situated well beyond the visible galactic disc which is detected using observations like rotation curves of galaxies. Current observations suggest that dark matter accounts for roughly 95% of the Milky Way’s mass, while the other 5% is accounted for with normal, baryonic matter. It is currently believed that dark matter played a significant role in the evolution of galaxies, initiating clumps of matter in the early Universe which eventually lead to structures like galaxies.
The galactic disc is the location of most of the interstellar medium (ISM, composed of gas and dust) and also contains a substantial amount of stars which orbit in roughly circular orbits confined to the galactic plane; it consists of most of the visible matter in the galaxy. The Sun is located about 8.5kpc (kiloparsecs) from the galactic center in the galactic disc. The spiral arms within the galactic disc are the main sites of star formation; the blue spiral arms are indicators of young, hot stars. Within the galactic disc is where most cosmic recycling occurs and as a result, the ISM has a higher metallicity than primordial gas. The metallicity-age relation is important for understanding galaxy evolution. Most open clusters are found in the galactic disc, but some globular clusters (roughly one third) are found here, as well. Open clusters are very young because they are loosely bound and are easily disrupted. Open clusters have high metallicities because they are formed from more recently formed ISM which has had time to become enriched with metals. In contrast, globular clusters are very old as they are strongly gravitationally bound. Since globular clusters consist of the oldest stars in the galaxy, the metallicity-age relation suggests that they should have very low metallicities but those found in the galactic disc have unusually high metallicities; this suggests that cosmic recycling enriches the contents of the globular clusters.
The stellar halo extends further than the galactic disc itself and is roughly spherical in shape. Here lie roughly two thirds of the galaxy’s globular clusters. The globular clusters found here, as expected, have very low metallicities because they are young and do not get contaminated by the cosmic recycling that takes place in the galactic disc. The stellar halo’s star formation has long since ceased; there is very little ISM left in the stellar halo. The stars within the stellar halo have largely elliptical and highly irregular orbital motions which are not confined to the galactic plane.
What Does Any of This Tell Us About Galaxy Evolution?
Without the dark matter halo, our galaxy wouldn’t be gravitationally bound the way it is today. With this, along with the fact that dark matter consists of 95% of the galaxy’s mass, it is suggested that dark matter played a key role in the initial formation of galaxies. The spiral arms of the galaxy suggest that the galaxy is rotating, which is well supported by observations. Older stars contained in the stellar halo having irregular, elliptical orbits and younger stars confined in the galactic disc having regular, circularized orbits suggests that the galaxy started out in a disordered state which over time settled down to a more orderly state; the galaxy originated as a large gas cloud that eventually collapsed and through the conservation of angular momentum, became a flat, rotating disc. The fact that globular clusters within the galactic disc have unusually high metallicities suggest that cosmic recycling is the origin; as time progresses, high mass stars explode and contaminate the surrounding ISM to further enrich future stars’ chemical composition. Furthermore, globular clusters which were once believed to originate from the Milky Way are now believed to having been captured by the Sagittarius Dwarf galaxy, which suggests that even though the galactic halo is the oldest component of the galaxy, it is still a dynamic part of the Milky Way. Currently, the Milky Way is in a collision course with its neighbour, the Andromeda galaxy. In about 4 billion years, the two spiral galaxies will collide to form what astronomers like to call the Milkomeda galaxy.
“This leaves us the rather discomforting conclusion that most of the inner part of the Galaxy including the Sun might not be saved from M31 after all, and the possibility of a merger in another 4 billion years cannot be ruled out.”
- Somak Raychaudhury and Donald Lynden-Bell
Monthly Notices of the Royal Astronomical Society 1989
The harsh reality of the distant universe with all of its violent interactions seems remote from our human existence and all might seem to be quiet and normal in our home the Milky Way. But it seems likely that in a mere 3 billion years, our neighbouring galaxy Andromeda and the Milky Way will fall together and have a close collision. They will likely merge and be reborn as a single giant elliptical galaxy over the course of another billion years or so. How might this metamorphosis play out and what might you see if you looked up at night over the next 4 billion years! The space between stars is so vast compared to their size that during a galaxy collision no individual stars actually collide with one another. So our sun and its family of planets will be taking a passive but exciting ride through the pair of coalescing galaxies and take on a spectacular view of the unfolding disaster in relative safety.
To examine the way the universe behaved in the past, astronomers look at extremely distant objects, such as supernovae in galaxies billions of light-years away. But how does that work? How can astronomers look out into space and see the universe back in time?
The answer lies in the speed of light. Light waves move very fast, about 186,000 miles per second (300,000 km/s). Light moves so fast that as you go about your daily life, it appears to travel instantaneously from one place to another. For example, it takes only a few billionths of a second for light to travel across your bedroom when you turn on a lamp.
In space, however, the distances are so immense that the time that light takes to travel is noticeable.
The Moon is Earth’s closest companion, at about 239,000 miles (390,000 km) away. Light takes around 1.3 seconds to travel that distance.
The Sun is 93 million miles (150 million km) away, far enough that the light it emits needs about 500 seconds to travel to Earth. We call the distance light takes to travel in a second a light-second, the distance it takes to travel in a minute a light-minute, and so on. So the Sun is about eight light-minutes away from Earth. The light shining on you right now first left the Sun eight minutes earlier.
Across our Milky Way galaxy, distances are measured in terms of how many years it takes light to travel. The nearest star is over four light-years away. So when we look at that nearest star, we see it not as it is today, but as it was four years ago. We are seeing the light that left that star four years previously and is just reaching us now.
The diameter of our galaxy is 100 thousand light-years. So when we look at even more distant stars, we see them as they were thousands to tens of thousands of years ago, depending on how far away they are and thus the distance their light has had to travel.
Galaxies are yet farther away in both space and time. Our nearest large neighbor galaxy, Andromeda, is about two and a half million light-years away. The Virgo Cluster of galaxies is the largest nearby collection of galaxies, at about 60 million light-years from the Milky Way. The light we see today from galaxies in the Virgo Cluster started on its path toward us at the same time as the age of the dinosaurs was ending on Earth. If you were in a Virgo Cluster galaxy today, and you had a telescope powerful enough to study Earth, you would be able to see the prehistoric reptiles.
Very distant galaxies are billions of light-years away. At that distance, their light tells what the universe was like billions of years ago. Since the age of the universe is about 14 billion years, these distant observations allow astronomers to measure changes over the lifetime of the universe. So when astronomers look out into space, they are essentially also looking back into time.
This fact was vital to the teams studying the expansion of space, because their goal was to compare the speed of the universe’s expansion in the past with the speed of the universe today. By studying extremely distant supernovae in faraway galaxies, they were able to judge the speed of the universe’s expansion in the early universe.
This composite image of M31 (also known as the Andromeda galaxy) shows X-ray data from NASA’s Chandra X-ray Observatory in gold, optical data from the Digitized Sky Survey in light blue and infrared data from the Spitzer Space Telescope in red. The Chandra data covers only the central region of M31 as shown in the inset box for the image.
New results show that the Chandra image would be about 40 times brighter than observed if Type Ia supernova in the bulge of this galaxy were triggered by material from a normal star falling onto a white dwarf star. This implies that the merger of two white dwarfs is the main trigger for Type Ia supernovas for the area observed by Chandra. Similar results for five elliptical galaxies were found. These findings represent a major advance in understanding the origin of Type Ia supernovas, explosions that are used as cosmic mile markers to measure the accelerated expansion of the universe and study dark energy. Most scientists agree that a Type Ia supernova occurs when a white dwarf star — a collapsed remnant of an elderly star — exceeds its weight limit, becomes unstable and explodes. However, there is uncertainty about what pushes the white dwarf over the edge, either accretion onto the white dwarf or a merger between two white dwarfs.
A Type Ia supernova caused by accreting material produces significant X-ray emission prior to the explosion. A supernova from a merger of two white dwarfs, on the other hand, would create significantly less. The scientists used the difference to decide between these two scenarios by examining the new Chandra data.
A third, less likely possibility is that the supernova explosion is triggered, in the accretion scenario, before the white dwarf reaches the expected mass limit. In this case, the detectable X-ray emission could be much lower than assumed for the accretion scenario. However, simulations of such explosions do not show agreement with the observed properties of Type Ia supernovas.
Read entire caption/view more images: chandra.harvard.edu/photo/2010/type1a/
Image credit: X-ray: NASA/CXC/MPA/M.Gilfanov & A.Bogdan; Infrared: NASA/JPL-Caltech/ SSC; Optical: DSS
Caption credit: Harvard-Smithsonian Center for Astrophysics
The Andromeda Galaxy (Messier Number 31) is probably the most famous galaxy. Because the distance to our home is relatively small (‘only’ about 2.5 million light-years) it appears as the largest spiral galaxy in the sky. It is thought that the structure of our own galaxy looks somewhat similar to M31. Messier 31 also has several satellit galaxies, the brightest ones are m32 and m110, seen on the picture beside.
In a clear and moonless night it is one of the farthest objects visible to the naked eye. About one trillion(!) stars let the galaxy appear as a faint smudge in the constallation of Andromeda. They also let the galaxy be the brightest messier galaxy: Apparent magnitude 3.4. Long exposed images reveal a diameter of more than six times the dimension of the full moon. A more recent study concludes that the Andromeda galaxy and the Milkyway galaxy are about equal in mass whereas the diameter of M31 is about 50 percent larger.
More information about this object can be found here.
See the proper motion of stars in this area here.
The universe hates you. Let’s get that out of the way right now. The universe loathes your guts and is infuriated by the way you dress, and the stupid way you talk sends it into a murderous rage. It’s just one bad morning and an empty coffee canister away from driving to your house and shanking you in the neck. With a supernova. It may happen tomorrow, or it may take billions of years. The universe is patient. It can wait. But rest assured: Some day, when you least expect it, it will reap a terrible vengeance from you. And it will go a little something like this:
Explanation: Is this what will become of our Milky Way Galaxy? Perhaps if we collide with the Andromeda Galaxy in a few billion years, it might. Pictured above is NGC 7252, a jumble of stars created by a huge collision between two large galaxies. The collision will take hundreds of millions of years and so is effectively caught frozen in time in the above image. The resulting pandemonium has been dubbed the Atoms-for-Peace galaxy because of its similarity to a cartoon of a large atom. The above image was taken recently by the MPG/ESO 2.2 meter telescope in Chile. NGC 7252 spans about 600,000 light years and lies about 220 million light years away toward the constellation of the Water Bearer (Aquarius). Since the sideways velocity of the Andromeda Galaxy (M31) is presently unknown, no one really knows for sure if the Milky Way will ever collide with M31.
Explanation: The big, beautiful Andromeda Galaxy, aka M31, is a spiral galaxy a mere 2.5 million light-years away. Two space-based observatories have combined to produce this intriguing composite image of Andromeda, at wavelengths outside the visible spectrum. The remarkable view follows the locations of this galaxy’s once and future stars. In reddish hues, image data from the large Herschel infrared observatory traces enormous lanes of dust, warmed by stars, sweeping along Andromeda’s spiral arms. The dust, in conjunction with the galaxy’s interstellar gas, comprises the raw material for future star formation. X-ray data from the XMM-Newton observatory in blue pinpoint Andromeda’s X-ray binary star systems. These systems likely contain neutron stars or stellar mass black holes that represent final stages in stellar evolution. More than twice the size of our own Milky Way, the Andromeda Galaxy is over 200,000 light-years across.
Andromeda (also known as M31) is the nearest large galaxy to our own Milky Way and is very similar to it in appearance. Studying Andromeda gives astronomers an external perspective on a galaxy much like our own—it’s like looking at a bigger sibling of our galaxy, said GuhaThakurta, an associate professor of astronomy and astrophysics. How such galaxies formed is a central question for astronomers, he said.
In spiral galaxies like Andromeda and the Milky Way, most of the prominent young stars lie in a flat disk with spiral arms. In addition, a spherical halo of scattered stars surrounds the disk.. a team of European astronomers reported a giant stream of stars threading through the halo of Andromeda. It is thought to be a vast trail of debris left over from an ancient merger of Andromeda with a smaller galaxy.