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When we look into the depths of the universe, what we see is not today's celestial bodies, but what they look like when the light that reaches the earth is emitted. The closest star to us is Proxima Centauri, which is about 4.24 light years away; in other words, what we see now is the light it emitted 4.24 years ago. However, for more distant stars, when we look back at them, we must also consider the expansion of the universe. Moreover, these stars were formed a long time ago. For example, Proxima Centauri was born 4.85 billion years ago, which is older than the sun.
How can we integrate the existing data to determine the age of all stars in the universe? We know that the universe has a history of 13.8 billion years, and the observable universe spans about 46.5 billion light years. So, what is the relationship between these two? When we observe a star, we can know its distance from us, but how do we know its age? This is a very good question. To answer this question, we need to put together two very different kinds of information. Below we will understand how astronomers do it. When we observe stars in very nearby universes, such as the Milky Way or many nearby galaxies, we can measure the properties of individual stars. Not only that, one of the attributes—the current distance of the star from the earth—is actually the same as the travel time of starlight. In other words, a star that is 4.24 light years away from us like Proxima Centauri has its light reaching our eyes after a full 4.24 years of space travel. However, these two pieces(hermes outlet) of information only apply to stars in the relatively nearby universe. When the observation distance is getting farther and farther, we can no longer distinguish the various properties of the stars one by one, because the line of sight of the telescope is leaving the local supercluster (also known as the Virgo supercluster, which includes the Milky Way and the Andromeda Galaxy Before the local group of galaxies, its resolution has gradually decreased. In addition, once we leave the local galaxy group, we must consider the expansion of the spatial structure itself, not only the extension of the light wavelength (causing red shift) but also the distance (in light years) of the object to be observed and the light travel time of the object ( In years). The first thing we need to understand is that when we look up at distant objects in the universe, we are actually looking back at the past. What is certain is that if you are observing stars that are several light years away, or even thousands or tens of thousands of light years away, it takes about the same number of "years" for their starlight to reach your eyes. But if you are observing galaxies tens of millions of light-years away, the expansion of the universe begins to have a huge impact. The reason is this: once the light leaves the light source, it will spread in all directions. Among them, the light that travels along the line of sight will eventually reach your eyes (to be precise the lens of the telescope), but before that, it must pass through all the space between you and the light source. It's a bit like putting some raisins in fermented bread; when the bread rises, the dough will expand and the raisins will move further apart. Stars that are relatively close at the beginning will only expand a little bit, while those that are far away at the beginning may become farther away as they propagate signals (such as light) to complete their journey. The fact that the universe is expanding means that the longer it takes for a star’s light to reach the earth, the greater the contradiction between its propagation time and our current distance from the star (in light years). Scientists already know the composition of the universe (ordinary matter, dark matter, and dark energy) and how fast the universe is expanding today. Therefore, we can perform the necessary calculations to determine how the universe has expanded throughout its history. This is a very powerful technique because it changes very little. In today's universe, as long as it is governed by general relativity, there is a clear relationship between the composition of the universe and its expansion speed over time. Scientists can measure the distances of various cosmic objects and their redshifts with unprecedented accuracy to determine this relationship, and confirm it in subsequent measurements of the cosmic microwave background and large-scale structure. This technology also means that when we observe an object in the universe, we can not only calculate how far back in time is, but also know how far the object is now from us. To give a few examples: •When the light of an object takes 100 million years to reach the earth, it means that we are seeing an object that is currently 101 million light years away from us; •When the light of an object takes 1 billion years to reach the earth, the object is now about 1.035 billion light years away from us; •If it takes 3 billion years for light to reach the earth, it means that this object is now about 3.346 billion light years away from us; •The light that arrives on the earth after 7 billion years comes from an object 9.28 billion light-years away from us; •The light that takes 10 billion years to reach the earth corresponds to an object that is 15.8 billion light years away; •The light that takes 12 billion years to reach the earth comes from an object about 22.6 billion light-years away. •The light from the most distant object detected so far, the GN-z11 galaxy, reached the lens of the Hubble Space Telescope after 13.4 billion years and is now about 32.1 billion light-years away. When measuring a distant object, we usually directly measure its brightness and its spectral redshift value, which is sufficient to determine its current distance and light travel time. When we measure light from objects that are 32.1 billion light-years away, we see light from 13.4 billion years ago, which was emitted 407 million years after the Big Bang. However, this is not enough to tell us the age of the stars in the galaxy; it can only tell us the age of light. In order to know the age of the star that produces this distant light, it is ideal to measure the exact properties of individual stars. We can do this with the stars in the Milky Way. With the highest resolution telescope, we can identify individual stars 50 million or 60 million light-years away. Unfortunately, this distance is only 0.1% between us and the edge of the observable universe; beyond this distance, we can no longer resolve individual stars. If we can measure a single star, we can construct the so-called color-magnitude diagram in astronomy: we can plot the relationship between the intrinsic brightness of a star and its color/temperature. This is very useful. When the stars first formed, their color-magnitude diagrams roughly appeared as a winding diagonal line, with the brightest stars also being the bluest and hottest, while the darkest stars were redder and colder. The youngest group of stars is a combination of stars of different colors/brightness. But as stars age, the hottest, bluest, and brightest stars consume fuel the fastest and begin to die out. They will eventually evolve into red giants and/or supergiants, but this means that the number of stars begins to evolve as the star ages. As long as we can distinguish individual stars in open star clusters, globular star clusters, or even nearby galaxies outside the Milky Way, we can accurately determine the age of a stellar population. Star family refers to a collection of stars in a galaxy whose age, chemical composition, spatial distribution and motion characteristics are relatively close. When you combine these data with information about the age of the received light, you can finally get the age of the star. However, what should we do when we can no longer observe individual stars in a galaxy? Is there any way to estimate the age of the stars inside the galaxy based on the observed light, even if we cannot distinguish the stars themselves? In fact, we can use an agent to obtain unobtainable information, but we need to sacrifice some accuracy when translating the age of stars inside the galaxy. When observing a distant object, such as a galaxy that cannot be resolved (or barely resolved), we can still measure the total starlight from all the stars in it. We can decompose this light into different wavelengths and determine how much of this light is ultraviolet light, blue light, green light, yellow light, infrared light, and so on. In other words, as long as the color of a distant galaxy is accurately measured, we can estimate the time of its most recent star formation and thus the age of the stars inside. However, since we must make these estimates, uncertainty is introduced. A galaxy that has undergone multiple star formation over hundreds of millions of years, and a galaxy that has only undergone an important merger and then formed all the stars at the same time, may present a completely different picture. For galaxies with extremely deep blue, the error may be as small as tens of millions of years, while for galaxies lacking young blue stars, the error may be as large as 1 billion to 2 billion years. Scientists can also apply other methods, such as measuring surface brightness fluctuations (this depends on variable stars, which are stars whose brightness fluctuates from the earth, and variable stars depend on the age of stars inside galaxies), but most methods are beyond It fails after a certain distance. However, if we can obtain spectroscopy measurement data instead of just measuring brightness through various color channels (that is, through luminosity), then we can get a little better result. By measuring the intensity of various atomic and molecular transitions by absorption lines and emission lines, we can determine the location of a stellar population based on the age since the most recent star formation explosion. To summarize, if you want to know the age of the star you are observing, you need to know two things: 1. You need to know how old the light you see is, which means you need to know how far this object is from the earth in the expanding universe; 2. You need to know the age of the star itself, starting from the moment you collect the starlight. When you can distinguish a single star, this is a very simple problem, but scientists can only distinguish a single star that is 50 to 60 million light years away. In contrast, the observable universe extends about 46 billion light-years in all directions, which means that we cannot use this method for the vast majority of stars in the universe. We can only use some indirect method, such as age estimation based on the color of the galaxy itself, but this will bring additional uncertainty. With a deeper understanding of stars and stellar evolution, as well as advanced instruments and telescopes that may be used in the near future, scientists are expected to learn more precisely about the most distant and oldest objects.
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