What is a Stellar Stream?
A stellar stream is a small galaxy close to a giant galaxy. Under the action of a strong gravitational tide, it gradually distorts, disintegrates, and is drawn into a slender and spectacular star stream.
Stellar stream
- Once a small galaxy approaches a giant galaxy, it will be subject to a strong gravitational tide, gradually twisting, disintegrating, and being drawn into a slender and spectacular star stream.
- Finding these interstellar immigrants requires sharp eyes. Theoretically, these stars will form a long string (that is, a star stream,
- The Saturn star stream discovered by Ibata and colleagues in 1994 is probably the most impressive known star stream. This stream of stars resembles a huge stellar necklace surrounding the Milky Way. It spans over 1 million light-years and contains approximately 100 million stars. It is connected to the Sagittarius dwarf elliptical galaxy. "Dwarf" means that the size of the galaxy is small. "Ellipse" means that the galaxy is ellipsoidal. ). There are 15 to 20 similar small galaxies around the Milky Way. They revolve around the Milky Way like satellites orbiting planets (hence the name satellite galaxies). These satellite galaxies vary in size, ranging from the Large Magellanic Cloud, which has a mass of about one tenth of the Milky Way, to the Sagittarius dwarf galaxy, which has a mass of one percent of the Milky Way, and even has a mass of only one million galaxies. One of the faint galaxies.
- Living near the Milky Way is not a comfortable thing. These small galaxies will gradually deform and eventually disintegrate. The Sagittarius dwarf galaxy has been dying for billions of years, and has now disintegrated. Its stars will be scattered throughout the galaxy. Such currents will gradually disappear, and future astronomers will have a hard time distinguishing them from the Milky Way's indigenous stars. Several other small galaxies are also being dismembered by the Milky Way, and some now have only one star stream (see table on page 81). Large Magellanic Clouds represent another, relatively rare phenomenon where the Milky Way takes gas, not stars.
- The mechanism behind the disintegration of galaxies is gravitational tidal action, which is very familiar to us. The same mechanism produces the ebb and flow of the oceans on Earth. Tidal forces occur when different parts of a celestial body experience different gravitational effects. The moon's gravitational pull on the side of the earth facing it is greater than the gravitational pull on the side facing away from it. Although the difference between the two forces is small enough not to smash the earth, it is enough to cause the ocean to swell slightly. As the two celestial bodies revolve around each other, the bulge of the ocean will also rotate, causing the sea level at a certain place on the earth to rise or fall periodically. Similarly, the Milky Way galaxy also exerts a greater gravitational effect on one side of a satellite galaxy or cluster than the other side, distorting them. With this effect, stars in satellite galaxies or star clusters will be gradually dragged away by the Milky Way (see the figure below). Over time, satellite galaxies will lose more and more stars. These stars, like crumbs falling from bread, form long tails before and after satellite galaxies.
- More subtle clues
- The star current will be gradually absorbed by the Milky Way and eventually become invisible. However, the subtle clues in the six-dimensional phase space can find the clues of these "interstellar immigrants" in the vast sea of stars.
- In this way, satellite galaxies such as the Sagittarius dwarf galaxy have contributed to the construction of the Milky Way. These discoveries completely changed astronomers' original theoretical understanding of galaxy formation: they used to think that all galaxies originated directly from the almost imperceptible fluctuations in the density of matter in the primitive universe. The density is almost the same, only about three ten-thousandths difference), and then it experienced early avalanche growth and soon evolved into what it is today. Now, based on observations of star currents, researchers generally believe that only dwarf galaxies with a mass not exceeding 1 billion times the sun have experienced such a rapid formation process; large galaxies such as the Milky Way galaxy have a mass equivalent to 100 billion suns. Formed by accretion and annexation of dwarf galaxies. This annexation process has continued to this day, but the intensity is much worse than before.
- Astronomers caught the galaxy's engulfing of neighboring galaxies, and they asked a deeper question: What is the chemical composition of these ancient galaxy "building materials"? What is the proportion of "interstellar immigrants" and "indigenous peoples" in large galaxies today? How did the chemical elements brought by these small galaxies change the early evolutionary history of the Milky Way? These galaxies, like fossils, record the history of galaxy construction. In addition, they can also play a significant role in detecting dark matter (see the short article on the next page).
- To understand these issues, astronomers need to know not only which stars are being looted by the Milky Way, but also which stars have already been captured. The difficulty for researchers is that once foreign stars and gas enter the galaxy, we cannot identify them by their unique spatial distribution. Astronomers must find more subtle clues to trace the origin of these stars, such as their movement patterns and some inherent characteristics of their chemical composition that are difficult to erase.
- We are all used to describing the movement of objects using position and speed. But motion has other different characteristics, which can be described by physical quantities such as energy and angular momentum. Just as the spatial position of an object can be described by three coordinates, it is called a three-dimensional space. We can also use the position plus momentum (a total of six physical quantities) to describe the motion state of an object. This abstract six-dimensional space is called Is the phase space. The advantage of phase space is that compared to real space, the arrangement pattern of stars in phase space is more reductive (here refers to the ability to recover to its original state after undergoing a huge change). Although the galaxy's annexation and digestion process usually destroys the spatial structure of the star current, it cannot erase their overall structure in phase space [this is an important principle in statistical mechanics-Liuou's theorem].
- Therefore, by measuring energy, angular momentum, and the density of random samples of stars in phase space, researchers can identify constellations that cannot be directly observed. They are the phantoms of satellite galaxies that have long been disintegrated by the Milky Way. Several research groups, such as Amina Helmi of the Kapteyn Astronomical Institute in Groningen, and Chris B. Brook of the University of Washington, USA Brook's own team has used this technology to discover the remains of some of the annexed satellite galaxies. The relics that have been found so far are located near the solar system, because the existing equipment cannot measure the three-dimensional motion of more distant stars with sufficient accuracy.
14 Stellar stream finds 14 unique star streams
- Scientists draw galaxy theoretical model similar to the Milky Way [1]
- Schruffman said that there are nearly 1,000 stellar streams in the 75,000 light-years of the Milky Way, and he estimates that each of the 14 stellar streams they observe is an independent structure. Another possibility is that stellar currents are actually smaller than estimated because they appear multiple times in different regions, so that researchers think the numbers are large.
- Columbia University researcher Kathryn Johnston described the stellar halo as "dough." "At the center of the Milky Way, clusters of stellar streams are clustered together, and you will see them mixed together uniformly. But when you look further away, you start to find independent streams, And features that are similar to pasta shells, which come from dwarfs in more elongated orbits. "
- According to Johnston, the gravitational tide of dwarf galaxies passing by with the Milky Way is stretched into a stream of stars like spaghetti. When stars run on the same orbital route at different speeds, these star streams Will fly around the galaxy wind pool battery. Rensselaer Polytechnic Institute Heidi Newberg and researcher Nathan Cole are trying to track some of these stars as they move around the Milky Way.
- Cole said: "Putting together these things is a huge challenge, because a stellar stream from one dwarf galaxy can orbit the Milky Way, passing through a stream of stars that have been torn from other dwarf galaxies." At least two sets of overlapping stellar streams were found in the direction of Virgo. Images of the Sloan Digital Sky Survey revealed a large number of stars in Virgo covering a vast area. Velocity test data can be used to separate overlapping structures, some from the tidal arm of the Sagittarius dwarf galaxy.
Stellar stream finds surviving dwarfs
- Data from the Sloan Digital Sky Survey also revealed 14 surviving dwarfs that accompany the Milky Way, two of which were announced at a seminar in Chicago. These small galaxies now operate in a halo of invisible dark matter, the gravitational pull of the dark matter holding the galaxy tightly together. Compared to dwarfs discovered before the Sloan Digital Sky Survey, the newly discovered dwarfs have weaker light.
- Although the Sloan Digital Sky Survey can detect extremely weakly lit dwarfs, this can only be done when such dwarfs are nearby, so hundreds of dwarfs may be hidden deep in the galaxy's halo. Johnston said, "The Sloan Digital Sky Survey has given us an in-depth understanding of the Milky Way and its neighbors. But we will still start a comprehensive map of the Milky Way, with many important discoveries hidden there for the next generation of exploration programs Reveals, for example, two new galaxy exploration programs to be implemented by Sloan Digital Sky Survey-III. " [2]
4015 Stellar stream 4 billion years ago
- On April 18, 2008, according to foreign media reports, an international team of astronomers discovered a giant stellar stream on the outer edges of two adjacent spiral galaxies. This is the first astronomer to make a panoramic observation of galaxy mutual engulfing, which is similar to the Milky Way engulfing the Sagittarius dwarf galaxy nearby.
- The first giant stellar stream was distributed around NGC 5907, a galaxy 40 million light years away from Earth. This giant galaxy was formed by the destruction of one of its dwarf companion galaxies at least 4 billion years ago. The research team said the dwarf galaxy lost most of its mass in the form of stars, clusters and dark matter. All of them are scattered around its orbit, and the most NGC 5907 eventually formed a complex cross-shaped "galaxy fossil" collection, with a radius of 150,000 light years.
- "Our observations provide a new perspective on the spectacular astronomical phenomena that occur around spiral galaxies. The halo contains 'Dwarf galaxy fossils' provide a special opportunity for us to study the final stages of galaxy-like galaxy formation. "Astronomers' search journey did not find the main objects of the galaxy being swallowed, and they concluded that the galaxy has now Completely destroyed. Martinez said: "These galaxies are difficult to detect and the density of the stars they contain is extremely low. This low density makes them feel terrifying. If linked to the death of dwarf galaxies, they could be Call it the ghost of a galaxy that has now been swallowed up. "
- During this observation, the research team also discovered another large and slender ring-shaped stellar stream in galaxy NGC 4013. NGC 4013 is located in the Ursa Major, nearly 50 million light years from Earth. This stellar stream stretches out from the center with a ghostly tail that is more than 80,000 light-years long and is made up of ancient metal-poor stars. Although its three-dimensional structure is unclear, it may be very similar to the "star current" of the unicorn constellation, which is a ring composed of stars surrounding the Milky Way, formed by the destruction of a dwarf galaxy 3 billion years ago.
- Jorg Parrubia, a theoretical astrophysicist and member of the research team at the University of Victoria in Canada, is adept at creating star flow models. "The theoretical model that matches these star currents allows us to reproduce their history and describe one of the most mysterious and controversial components of the galaxy-dark matter," Parlubia said.
- Martinez said that in order to complete the search and detection of the star current, the research team invited the famous astronomical photographer Jay Jabani to join. Jabani's help played a decisive role in obtaining the pictures. It turns out once again that amateurs can make a huge contribution to astronomical research. For years, Jabbani has facilitated the use of small automatic telescopes in New Mexico and Australia to obtain fascinating color deep-space pictures. His pictures have been published in the most popular astronomy magazine in the world. Jabani's performance in this observation proves that amateur astronomers can make potential contributions to astronomy research in the 21st century. With the help of new technologies, they have the ability to participate in highly competitive international scientific research projects. [3]
3 3 stellar streams surround the Milky Way
- In 2007, American scientists discovered three stellar streams around the Milky Way, which could be the remains of "dismembered" star clusters and galaxies.
- The discovery of stellar streams suggests that the Milky Way may be a dangerous place for these "hurried passengers". Researcher Carl Griermeier of the California Institute of Technology has described their findings in detail at a meeting of the American Astronomical Society. These stellar streams are thought to have formed billions of years ago, when the Milky Way's gravitational pull slowly tear apart globular clusters and even dwarf galaxies. The distance between these stars used to be very tight, but when the galaxy halo passed at high speed, under the gravitational force of the Milky Way, their speed slowed down, and the distance between them was several light years. In this study, Greermeier and colleagues analyzed data from the Sloan Digital Sky Survey (SDSS) program, compared star colors and luminous intensities, and grouped similar stars into groups. It turned out that two of the newly found star streams, about 13,000 light-years away from Earth, could be the remnants of globular clusters consisting of hundreds of thousands of ancient stars. Astronomers have identified about 150 globular star clusters orbiting the Milky Way, but they thought that there could have been thousands of such clusters in the universe in the past.
- The third stellar stream is about 130,000 light-years away from Earth and may be the dwarf galaxy currently known to be closest to the Milky Way. So far, astronomers have found about 20 dwarf galaxies in the Milky Way, but at the same time they are confused about why they cannot find more dwarf galaxies. "The new findings may have revealed to us why they can't be seen, as they are all similar to these stellar streams mentioned above," Greermeier said in an interview with NASA's Space Network. He went on to explain There may be a safe distance between the dwarf galaxy and the Milky Way galaxy, and any dwarf galaxy that crosses the thunder pool may be "scorched".
- Astronomers believe that the dwarf galaxy is composed of hundreds of millions of stars and may be filled with dark matter, which is considered to be the "glue" that holds the Milky Way tightly together. According to Greermeier, stellar currents will be the focus of attention in the scientific community in the next few years, because they are a window for scientists to understand the past, present, and future of the Milky Way, and may be evidence to prove the existence of dark matter.
- A theory about the formation of the Milky Way says that countless dwarf galaxies have fused together and continued this fusion trend, eventually forming the Milky Way. Scientists speculate that the newly discovered dwarf galaxy may be the result of this fusion, gradually being surrendered to the galaxy under the temptation of the galaxy's gravity. "The latest discovery is an exciting time for archaeological research in the galaxy, and discovering more stellar streams does help us better understand the structure and evolution of the galaxy," Griermeier said.
11 11 new stellar streams in the galaxy
- According to a report by the New Scientist website on August 16, 2008, a large student in California recently discovered 11 new stellar streams in the Milky Way. Some scientists believe that the discovery provides new information for the existence of "violent events" in the universe. evidence.
- Kevin Schlaufman, a student from the University of California, USA, observed the rotation rate of halo stars in the galaxy and found 11 new stellar streams. Previously, there were only two stellar streams in this area. These 11 stellar streams are all moving at about the same speed, which indicates that they come from the same place and are likely to be swallowed by our galaxy from the dwarf galaxies nearby.
- In this study, Kevin and his mentor, Professor Connie Rockosi, analyzed data from the Sloan Digital Sky Survey program. They found that there is an area in our galaxy with a range of 75,000 light-years, and there are about 1,000 stellar streams in this area, all of which our galaxy has plundered from other galaxies. Teacher Connie said that this shows that "violent incidents" in the universe are frequent.
- Professor Connie introduced that once a small galaxy approaches a giant galaxy, it will be gradually distorted, disintegrated, and pulled into a slender and spectacular star stream by the strong gravitational tide of the Milky Way. The discovery of these 11 stellar streams indicates that our galaxy is very "violent", but this is very common in the cosmic species, and behind the "violent events" of the cosmic species, gravitational tides are at work. This gravitational tidal effect also affects the earth, such as the ebb and flow of the oceans on the earth. [4]