The universe is divided into the observable universe and the unobservable universe. The observable universe contains visible matter and invisible matter, and the most intuitive carrier of visible matter is the stars in the sky.
So how many stars are there in the observable universe? Let’s count them today.
However, what can be roughly estimated at present can only be celestial bodies above the planetary level. Celestial bodies smaller than planets and cosmic dust are not included. It is really impossible to count, even in the solar system alone.
The picture comes from the Internet, the observable universe
First, let’s talk about what the “observable universe” is.
According to the current mainstream Big Bang theory, the universe is finite, but we cannot see the entire universe. The part that can be seen is called the "observable universe".
Because the expansion speed of distant space has exceeded the speed of light, the light from distant stars has not had time to reach the earth, so we cannot see them, nor can we accurately calculate their number.
It is currently believed that the scope of the observable universe is a sphere with the Earth as the center and a radius of approximately 46.5 billion light years.
Pictures come from the Internet, the extragalactic galaxy Andromeda Galaxy
Since Hubble discovered Andromeda Nebula and Triangulum Galaxy the distance from the earth far exceeds Milky Way diameter and concluded that they are " extragalactic galaxies " , people learned that there are more galaxies beyond the Milky Way.
Now we know that there are four extragalactic galaxies visible to the naked eye in the night sky, namely Andromeda , Triangulum Galaxy, Large Magellanic Cloud and Small Magellanic Cloud.
How many galaxies are there in the observable universe?
The picture comes from the Internet, Hubble Super Deep Field
Astronomers believe that in the large-scale universe, the distribution of galaxies is relatively uniform, so they use telescopes to photograph a small area and estimate the number of galaxies in it, and then know that this area is distributed throughout the entire universe. The area ratio of celestial sphere can be used to estimate the number of galaxies in the entire observable universe.
Based on the density of galaxies observed by the Hubble Telescope , astronomers estimate that there are approximately two trillion galaxies in the observable universe.
Now that we know the approximate number of galaxies, let’s estimate the number of stars and planets.
Stars are easy to talk about and can be observed through telescopes.
Galaxies vary in size. It is estimated that each galaxy has an average of 200 billion stars, so the observable universe has a total of about forty trillion stars.
Planets are not so simple, because planets are difficult to observe directly.
The picture comes from the Internet, simulating a binary star system
About a quarter of the stars in the universe are single star systems similar to the solar system, and most of the stars are binary or multi-star systems.
According to the existing stellar theory, there are more planets in single star systems , while there are fewer planets in binary or multi-star systems.
Astronomers used the Kepler telescope to observe exostars and found that 410 stars have a total of 700 planets. However, this is not an estimate, but a theoretical lower limit.
Because the planets observed are those that are larger, closer to the star, and just at a suitable observation angle.
Pictures from the Internet
With the advancement of science and technology, astronomers can estimate this number more accurately. For now, the number of planets is at least eight times the number of stars .
Therefore, the conclusion is drawn. In our observable universe, there are at least two trillion galaxies, and there are at least 40 trillion stars. The number of planets is at least or more than 3 trillion , with an order of magnitude of 10 ^24 to 10^25.
Okay, let’s stop talking today. You may think that the number is beyond imagination, but as mentioned at the beginning, these stars are only a very small part of the matter in the entire universe, and the invisible matter accounts for a much larger proportion.
How much matter there is in the unobservable universe is still unknown.
Author: Lian Wei
Copyright: Shangzhi Astronomy Popular Science Communication Center
