US, WASHINGTON (ORDO NEWS) — Looking into the night sky, it is quite difficult for an amateur astronomer to calculate the number of stars visible to the naked eye. With larger telescopes, more stars become visible, which makes counting impossible due to the amount of time it takes. So how do astronomers figure out how many stars in the universe?
“The first unpleasant part is an attempt to determine what the universe means,” said David Kornreich, associate professor at Ithaca College in New York State.
“I don’t know the answer, because I don’t know whether the universe is infinitely large or not,” he said. The observable universe takes us back about 13.8 billion years ago, but other than that, we could probably see a lot more. Some astronomers also think that we can live in a “multiverse”, where there would be other universes, like ours, contained in some larger entity.
The simplest answer may be to estimate the number of stars in a typical galaxy, and then multiply this by the estimated number of galaxies in the universe. But even this is difficult, as some galaxies are better visible in the visible part of the spectrum, and some in the infrared. There are also barriers to assessment that need to be overcome.
In October 2016, an article in Science (based on images of deep space taken by the Hubble Space Telescope) suggested that there are about 2 trillion galaxies in the observable Universe, or about 10 times more galaxies than previously thought. Christopher Conselis, a professor of astrophysics at the University of Nottingham in the UK, said there are about 100 million stars in the middle galaxy.
However, telescopes cannot see all the stars in the galaxy. The Sloan Digital Sky Survey estimate, which catalogs all observed objects in a third of the sky, in 2008 showed that about 48 million stars are about half what astronomers expected to see. Thus, many astronomers estimate the number of stars in a galaxy based on its mass, an estimate of which has its difficulties, since dark matter and the rotation of the galaxy must be filtered out before making an estimate.
Missions such as Gaia (the European Space Agency‘s space probe), launched in 2013, may provide additional answers. Gaia aims to accurately map about 1 billion stars in the Milky Way. The probe is based on the previous Hipparchus mission, which accurately detected 100,000 stars, and also mapped 1 million stars with less accuracy.
“Gaia will follow each of its 1 billion target stars, 70 times over a five-year period, accurately plotting their position, distances, movements and changes in brightness,” the ESA said on its website. “Together, these measurements will create an unprecedented picture of the structure and evolution of our galaxy. Thanks to such missions, we are one step closer to a more reliable assessment of the question that is often asked: “How many stars are there in the universe?”
Even if we narrow down the definition to an “observable” universe — what we can see — estimating the number of stars in it requires knowledge of how large the universe is. The first difficulty is that the universe itself is expanding, and the second complexity is that space-time can be curved.
As a simple example, light from the farthest objects from us can fly to us about 13.8 billion light years. Thus, the radius of the observable Universe should be 13.8 billion light years.
Galactic observations of a Star are easier to count when they are inside the galaxies, since it is there that they tend to congest. To even begin to estimate the number of stars, you will need to estimate the number of galaxies and get some average value.
According to some estimates, the stellar mass of the Milky Way has 100 billion “solar masses”, or 100 billion solar masses. Averaging the types of stars in our galaxy, this would yield approximately 100 billion stars in the galaxy. However, this value can vary, depending on how many stars are larger and smaller than our own sun. In addition, according to other estimates, the Milky Way may have 200 billion stars or more.
The discovery in 1995 of a small spot in Ursa Major revealed about 3,000 faint galaxies. In 2003-2004, using modernized tools, scientists looked at a smaller point in the constellation Pecs and discovered 10,000 galaxies, but a more detailed study in 2012 using even more advanced instruments showed about 5,500 galaxies.
Kornreich used a very rough estimate – 10 trillion galaxies in the universe. Multiplying this by 100 billion stars of the Milky Way leads to a large number: 1,000,000,000,000,000,000,000,000,000 or “1” with 24 zeros (1 24 or 1 quadrillion stars). Kornreich stressed that the number is probably a serious underestimation, since a more detailed look at the Universe will reveal even more galaxies.
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