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I have an assignment in school where I have to estimate the magnitudes and colors of certain stars in certain constellations. I know that some stars in some constellations are brighter and considered either blue, yellow, or red depending on their temperature, but I was wondering if these magnitudes and temperatures are always going to be the same for those stars. For example if one star in a constellation is say 9,000k and 3.9 Magnitude, will it always be that? Or will it vary at all?

A star will spend most of its lifetime turning hydrogen into helium in its core; this nuclear reaction releases energy and makes the star shine. During this phase, a given star's colour and magnitude remain essentially the same (we call this phase of a star's life the main sequence phase). Towards the end of its life, however, after the star runs out of hydrogen to burn into helium, the star's outer layers expand and cool down; we call this its red giant phase. Since the temperature of the star's outer envelope decreases, its colour changes - it becomes redder (hence the name red giant). Eventually, the star's core will become hot enough to fuse helium into carbon and will become bluer than it was before. Indeed, the end of a massive star's life is sequence by a series of envelope expansions and burning of heavier elements, which cause the colour of the star to redden and become bluer respectively. The luminosity of the star changes along with the temperature, so the magnitude of the star will change as well. Remember, however, that this period of colour and magnitude change is short, only about 10% of the star's total lifetime. So it is reasonable to say that stars have a roughly constant magnitude, colour and temperature during their lifetimes.

#### Kristine Spekkens

Kristine studies the dynamics of galaxies and what they can teach us about dark matter in the universe. She got her Ph.D from Cornell in August 2005, was a Jansky post-doctoral fellow at Rutgers University from 2005-2008, and is now a faculty member at the Royal Military College of Canada and at Queen's University.

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