What determines how bright a star is?

Answer 1

three factors, the size of the star, how far away it is and how hot it is.

Answer 2

Well stopxsnap you have got a lot of answers here haven't you.
braxton_paul and salient2 gave good answers. The short answer is size, temperature and distance. A more complete answer is below.

APPARENT MAGNITUDE
If you go outside on a dark night you can use a flashlight to see the path ahead of you. There may be airport runway lights faintly visible from miles away, but they are to dim to light your way. Does that mean that your little flashlight is brighter than the runway lights? Well a scientist would say that the "absolute" magnitude of the flashlight would be less than the airport, but the "apparent" magnitude of the flashlight would be greater than the airport. It is the same with stars. The sun is so bright that it can burn our skin, but the star Spica is actually MUCH brighter(absolute magnitude) than the sun, but very nuch farther away so it appears (apparent magnitude) very dim to us.

ABSOLUTE MAGNITUDE
Is a measure of a stars intrinsic brightness. How bright would it appear to an observer if it were side by side with other stars if all the stars were the same distance away.

STELLAR MASS
All stars form from the gradual collapse of a cloud of gas and dust. The mass of the star that forms from that collapse determines its entire future. The greater the mass of the star the hotter it will burn and the hotter an object the brighter it will be.

STELLAR SIZE
At first you might think that the larger a stars mass the bigger that it will be. Well; that is true to a point. Two stars with nearly the same mass can have very different sizes because they are at different ages. Stars shrink and expand at various times throughout their lives. And the larger the surface area the more light that is given off.

STELLAR TEMPERATURE
If two objects are the same size THEN the hotter object will always be brighter than the cooler object. Just like stellar size discussed above, a stars temperature varies in a well defined way(predetermined by its initial mass at birth) from its birth until its death as shown on the H-R Diagram

STELLAR AGE
Have you ever spent time around a camp fire? When you first start the fire it is very dim as the twigs start to catch fire. Over the next several minutes it gets brighter and brighter and finally so hot that you have to move away from the fire. After an hour or so it begins to burn down until finally only a few dim red embers remain. Stars are the same in that they go through periods of brightening and dimming several times in their lives. These changes in brightness can be because of a change in temperature or a change in size or both. Their brightness varies throughout their lifetime in a very exact way that scientists understand and have plotted on the "H-R Diagram"

ISM
The interstellar medium (ISM) is the gas and dust that is present everywhere in the galaxy. If a cloud of ISM happens to get between you and a star it will make the star appear dimmer than it otherwise would be. An extreme example of this is the "Coal Sack" only visable from the southern hemisphere. It is a cloud of ISM so thick that nothing can be seen through it. This is a change in the "apparent" magnitude not the "absolute" magnitude of a star.


STELLAR COMPOSITION
When the universe was young the first stars that formed were made almost entirely of hydrogen. These stars are known as "Population II" stars. Every succeeding generation of stars has higher and higher ratios of heaver elements. This changing composition affect stelar brightness and evolution though not nearly as much as the previous subjects.

Answer 3

Mass determines brightness as rated on an H-R Diagram. The more mass a star has, irregardless of temperature, the more absolute brightness it has. There are cool, bright stars as well as hot, bright stars. Brightness is relative to mass. Also, the larger the mass, the bigger its core,; the bigger its core, the more hydrogen (fuel) it burns during nuclear fusion.

Answer 4

Generally speaking, the star's size and temperature determine its brightness.

There are two ways to list a star's brightness -- absolute magnitude and apparent magnitude.
(1) absolute magnitude...how bright any star would be if observed from exactly 32.6 light years away;
(2) apparent magnitude...how bright any star appears when observed from Earth.

Answer 5

The star is bigger than others or brighter

Answer 6

the newer the star, the brighter it is

Answer 7

How rapidly nuclear fusion happens at its core (i.e., how much energy it produces per second), how large (radius) it is, and how far away from us it is.

Answer 8

its heat and distance from earth. the hotter it will looks blue, the cooler red. also the nearer blue,the further red.

Answer 9

http://physics.bgsu.edu/astroweb/the_best_and_the_brightest.html