How do astronomers calculate the distance of stars and galaxis that are light year far, their surface temp, and their mass?

Question

I was searching for Andromeda galaxy and many stars, but i realised how do they calculate the distance from earth? like, andromeda galaxy is 2.5 MILLION LIGHT YEARS far from earth.
Sirius has surface temperature as 9,940k,
its mass is 4.018 x 10^30kg.

But the question is how do they calculate it??

Answer 1

the temperature is relatively easy because it functions like black body radiation. The energy (the wavelengths of the light, the non-line spectrum or background light) that is emitted varies as a function of temperature. We can observe the energy distribution.



Size and distance are a little more difficult and require some assumptions about the behavior of the universe as a function of time. In an expanding universe, light emitted from a star will "red-shift" (doppler effect), and how much the light shifts from that is taken to change with distance (because distance from here is equivalent to time). The precise shift can be measured from the line spectrum, which is well defined for the main element emissions and serves as what should be seen if there is no relative motion. But using that method does require first establishing how expansion changes with time, and that came (initially) from the work of Hubbell.



Naturally, if you know the distance to an object, you can convert the observed size into a real size (a volume). Conversion of a volume into a mass requires making assumptions about density, and that can come from the specific spectrum of the light (the line spectrum will indicate what is present in the star) and by analogy to similar nearby objects that allow a better idea of mass-volume-light spectrum relationships for other reasons. Also, parallax can be used for relatively close objects to define distance, so that helped establish the general relationships between volume, mass, and light intensity and spectrum.



It is a lot more complicated than that in practice, but those are the basic ideas. And there are a lot of measurement precision considerations and correlation precision factors, so there is a large uncertainty.



All of those calculations depend primarily on the rate of the expansion of the universe, and that has been worked on extensively and become fairly well defined.

Answer 2

So, local stars will radiate at peak temperatures. The sun is one we call a 'yellow dwarf', while Sirius is a bit more toward the blue end of the spectrum. It's peak light is hotter than the sun's - which means it has a certain mass, and size.

If you know it's luminosity or mass or color, you can look at the HR diagram to figure out the other characteristics:

http://lcogt.net/files/styles/fourcol-image/public/spacebook/HR%20Diagram.png



To measure the distance to galaxies, there's a couple of methods, but the most convenient and common is to look for Cephid Variable stars; These stars brighten and dim in a known, predictable way. There is a relationship to their mass and brightenss with the rate at which they brighten and dim. They're sometimes called a 'standard candle' - we know how bright they really are, so how bright they *appear* in another galaxy tells us how far away they are - and, that gives us the distance to that galaxy.

Answer 3

It's not always easy to calculate these things, and astronomers have developed clever techniques to measure them.



Distances: distance is a very tough thing to calculate in astronomy, but there are various ways of doing it. If a star is very close to Earth (a couple hundred light-years with modern technology), the most accurate technique is something called the "parallax"- as the Earth orbits the sun, this causes the stars to move very slightly in the sky, and by measuring that effect we can determine how far away they are.



For objects further away, the parallax doesn't work, so basically what you need to do is find a way to figure out how bright the star is, and compare this to how bright it appears. There are once again various tricks to figure this out (e.g. for distant galaxies, very special supernovae called type 1a supernovae always reach the same brightness so we can use that). There are lots of different methods, each with their own pros and cons.



Surface temperature: actually this one is pretty easy. What you do with this one is look at the wavelengths that star is emitting, and see where in the spectrum that it is brightest. This is known as the "black body" phenomenon and it lets you measure surface temperature quite accurately.



Mass: this one is again a bit harder. Basically what you need is for the star in question to be close enough to another star that they orbit each other- fortunately most stars do this. By measuring how long it takes each one to orbit around the center of mass, you can measure how much the star weighs.



So there are various ways to calculate these things. It's very difficult, though, and lots of things can go wrong. There's a bit of a joke among other fields of science that if a theory is on the same order of magnitude as the measurements (meaning that theory is sort of around the same numbers as the measurement), the astronomers will accept that as good enough.

Answer 4

It's not always easy to calculate these things, and astronomers have developed clever techniques to measure them.