(ORDO NEWS) — The Milky Way is the galaxy in which the Earth is located. Part of it is visible on a clear night from Earth as a thick band of stars stretching across the sky. We can see thousands of these stars with the naked eye, and many more with a telescope. But how many stars are in the Milky Way?
“This is an amazingly difficult question. You can’t just sit and count the stars in the galaxy,” said David Kornreich, an assistant professor at a college in New York.
Even in the Andromeda Galaxy – which is bright, large, and relatively close to Earth, at 2.3 million light-years – only the largest stars and a few variable stars (especially the Cepheid variables) are bright enough to be seen in telescopes from such distances. For example, a star the size of the sun would be too hard to see.
Structure of the Milky Way
From observations, astronomers know that the Milky Way is a closed spiral galaxy that is about 100,000 light-years across.
Viewed from outside the galaxy, one could see a central bulge surrounded by four arms, two large and two smaller. The main arms of the Milky Way are known as Perseus and Sagittarius. The sun is in one of two small arms called the Orion arm.
The galaxy also has a huge halo of hot gas around it, several hundred thousand light-years across. Astronomers have calculated that the halo is as massive as all the stars in the Milky Way. However, many of the stars in the Milky Way are hard to see.
This is because at the center of the galaxy is the Galactic Bulge filled with stars, gas and dust, as well as a supermassive black hole.
This region is so densely filled with material that even powerful telescopes cannot see through it. Astronomers aren’t sure when or how the bulge formed; some suggest that the Milky Way’s early history was changed when the galaxy collided with another.
Between galaxies with the same mass, there can be differences in the types of stars. Kornreich cautioned that it was very difficult to talk about this in general, but said that the difference could be made when considering elliptical galaxies and spiral galaxies.
Elliptical galaxies tend to have more K and M type red dwarfs than spiral galaxies. Because elliptical galaxies are older, they will have less gas because it was blown away during their evolution.
Once the mass of a galaxy is determined, another trick is to figure out how much of that mass is in the stars. Most of the mass will be made up of dark matter, a type of matter that does not emit light, but which is believed to make up most of the mass of the universe.
You have to model the galaxy and see what percentage of that mass is in the stars, Kornreich said. In a typical galaxy, if you measure its mass, about 90 percent of it is dark matter.
Kornreich calculated that since most of the remaining “matter” in the galaxy is scattered gas and dust, about 3 percent of the galaxy’s mass will be composed of stars, but that could change. Further, the size of the stars themselves can vary greatly – from the size of our Sun, to something ten times smaller or larger. The number of stars is about the same…
So is there any way to figure out exactly how many stars there are in a galaxy? According to one calculation, the Milky Way has a mass of about 100 billion solar masses, or 100 billion solar mass stars.
This includes stars that are larger or smaller than our Sun, which averages them out. However, the mass of the galaxy is difficult to calculate – other estimates put the mass of the galaxy at between 400 and 700 billion solar masses.
The European Space Agency’s Gaia mission is mapping the location of approximately 1 billion stars in the Milky Way. The ESA says Gaia will map 1 percent of the Milky Way’s stellar population, giving us an estimate of the total number of stars in our galaxy at 100 billion. Gaia’s goal is to create the world’s best 3D map of the Milky Way.
But Kornreich said those numbers are approximations. More advanced models can make the approximation more accurate, but it would be very difficult to count the stars one by one by hand and tell you exactly how many there are in the galaxy.
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