The Milky Way is the galaxy by which the Earth resides. Part of it’s seen on a transparent night time from Earth, as a thick band of stars stretching throughout the sky. We can see hundreds of those stars with the bare eye, and plenty of extra in a telescope. But what number of stars are within the Milky Way?
“It’s a surprisingly difficult question to answer. You can’t just sit around and count stars, generally, in a galaxy,” stated David Kornreich, an assistant professor at Ithaca College in New York. He was the founding father of the “Ask An Astronomer” service at Cornell University.
Even within the Andromeda Galaxy — which is vibrant, giant and comparatively shut by Earth, at 2.3 million light-years away — solely the biggest stars and some variable stars (notably Cepheid variables) are vibrant sufficient to shine in telescopes from that distance. A sun-size star could be too troublesome for us to see. So astronomers estimate, utilizing a few of the strategies under.
The Milky Way’s construction
From observations, astronomers know that the
Milky Way is a barred spiral galaxy that’s about 100,000 light-years throughout. A view outdoors the galaxy would reveal a central bulge surrounded by 4 arms, two main and two minor. The Milky Way’s main arms are generally known as Perseus and Sagittarius. The sun is situated in certainly one of two minor spurs, which is named the Orion Arm.
The galaxy additionally has an enormous hot-gas halo round it that’s a number of a whole bunch of hundreds of light-years in diameter. Astronomers estimate that the halo is simply as huge as the entire stars within the Milky Way. Many of the Milky Way’s stars, nevertheless, are powerful to view. That is as a result of the middle of the galaxy has a galactic bulge stuffed with stars, fuel and dust — in addition to a supermassive black hole. This space is so thick with materials that even highly effective telescopes can not see by means of it. Astronomers
aren’t sure when and how the bulge formed; some recommend that the Milky Way’s early historical past was modified when the galaxy collided with one other one.
Astronomers used to assume that the entire stars within the universe have been situated within the Milky Way, however that modified within the Nineteen Twenties. Astronomer Edwin Hubble used a star known as a Cepheid variable to measure distances within the sky. From there, astronomers realized that there have been complete galaxies within the universe separate from the Milky Way.
The major means astronomers estimate stars in a galaxy is by figuring out the galaxy’s mass. The mass is estimated by taking a look at how the galaxy rotates, in addition to its spectrum utilizing spectroscopy.
All galaxies are transferring away from one another, and their gentle is shifted to the purple finish of the spectrum as a result of this stretches out the sunshine’s wavelengths. This is named “
redshift.” In a rotating galaxy, nevertheless, there shall be a portion that’s extra “blueshifted” as a result of that portion is barely transferring towards Earth. Astronomers should additionally know what the inclination or orientation of the galaxy is earlier than making an estimate, which is typically merely an “educated guess,” Kornreich stated.
A way known as “long-slit spectroscopy” is finest for performing the sort of work. Here, an elongated object equivalent to a galaxy is seen by means of an elongated slit, and the sunshine is refracted utilizing a tool equivalent to a prism. This breaks out the colours of the celebrities into the colours of the rainbow.
Some of these colours shall be lacking, displaying the identical “patterns” of lacking parts as sure parts of the periodic desk. This lets astronomers determine what parts are within the stars. Each sort of star has a singular chemical fingerprint that might present up in telescopes. (This is the premise of the OBAFGKM sequence astronomers use to differentiate between sorts of stars.)
Any form of telescope can do that form of spectroscopy work. Kornreich typically makes use of the 200-inch telescope on the Palomar Observatory on the California Institute of Technology, however he added that just about any telescope of adequate measurement could be ample.
The ultimate could be utilizing a telescope in orbit as a result of scattering happens in Earth’s ambiance from gentle air pollution and in addition from pure occasions — even one thing so simple as a sundown. The
Hubble Space Telescope is one observatory recognized for this form of work, Kornreich added. A successor observatory known as the James Webb Space Telescope is predicted to launch in 2020. The problem, nevertheless, is that Hubble is a telescope in excessive demand – and the identical is predicted of Webb after its launch. So the observatories cannot spend all of their time estimating galaxy mass. How a lot of the mass is stars?
Between totally different galaxies of the identical mass, there may very well be variances as to the sorts of stars and the general mass. Kornreich cautioned this could be very arduous to talk about typically, however stated that one distinction may very well be taking a look at elliptical galaxies vs. spiral galaxies equivalent to our personal, the
Milky Way. Elliptical galaxies are inclined to have extra Okay- and M-type red dwarf stars than spiral galaxies. Because elliptical galaxies are older, they are going to have much less fuel as a result of that was blown away throughout their evolution.
Once a galaxy’s mass is decided, the opposite tough factor is determining how a lot of that mass is manufactured from stars. Most of the mass shall be made up of
dark matter, a kind of matter that emits no gentle however which is believed to make up many of the mass of the universe.
“You have to model the galaxy and see if you can understand what the percentage of that mass of stars is,” Kornreich stated. “In a typical galaxy, if you measure its mass by looking at the rotation curve, about 90 percent of that is dark matter.”
With a lot of the remaining “stuff” within the galaxy made up of diffuse fuel and dust, Kornreich estimated that about 3 % of the galaxy’s mass shall be made up of stars, however that would differ. Further, the dimensions of the celebrities itself can enormously differ from one thing that’s the measurement of our sun, to one thing dozens of occasions smaller or greater.The variety of stars is roughly …
So is there any means to determine what number of stars are for positive? In the tip, it comes right down to an estimate. In one calculation, the Milky Way has a mass of about 100 billion solar plenty, so it’s best to translate that to 100 billion stars. This accounts for the celebrities that might be greater or smaller than our sun, and averages them out. However, the mass is hard to calculate — different estimates have stated the galaxy has a mass of between 400 billion and 700 billion solar plenty.
The European Space Agency’s Gaia mission is mapping the areas of roughly 1 billion stars within the Milky Way. ESA says Gaia will map
1 percent of the stellar content in the Milky Way, which places the estimate of the total stars in our galaxy at 100 billion. Gaia’s objective is to make the all-time three-dimensional map of the Milky Way.
The caveat, Kornreich stated, is that these numbers are approximations. More superior fashions could make the approximation extra correct, however it will be very troublesome to depend the celebrities one after the other and inform you for positive what number of are within the galaxy.