What does a star's color tell us about it?

Its chemical composition — different elements produce different colors Its age — young stars are blue and old stars are red Its surface temperature — blue stars are hottest, red stars are coolest Its distance from Earth — nearby stars look blue and distant stars look red

Approximately what surface temperature does a red star like Betelgeuse have?

About 100 Kelvin — red stars are cold About 3,000 to 4,000 Kelvin About 15,000 to 20,000 Kelvin About 100,000 Kelvin

What is luminosity in the context of stars?

How bright a star appears to an observer on Earth The total amount of energy a star radiates into space per second — its true power output The amount of visible light a star emits, not counting other wavelengths How long a star has been shining

What is the difference between apparent magnitude and absolute magnitude?

Apparent magnitude measures visible light; absolute magnitude measures all wavelengths Absolute magnitude is how astronomers estimated brightness before telescopes; apparent magnitude is the modern measurement Apparent magnitude is how bright a star looks from Earth; absolute magnitude is how bright it would look from a standard distance of 10 parsecs They are the same measurement — just different names used by different countries

Sirius is the brightest star in our night sky. Is Sirius also the most luminous star in the Milky Way?

Yes — the brightest-looking star is always the most luminous No — Sirius appears brightest because it is relatively close (8.6 light-years), not because it is the most intrinsically luminous star Yes — Sirius is a blue supergiant, the most luminous type of star There is no way to know — luminosity cannot be measured

Betelgeuse is a red supergiant star and appears dimmer than Sirius in our sky, yet it is far more luminous. How is this possible?

Betelgeuse is cooler than Sirius, so most of its energy is in infrared not visible light Sirius is actually more luminous — Betelgeuse only appears bright in certain wavelengths Betelgeuse is much farther away (about 700 light-years versus 8.6 for Sirius), so its enormous luminosity is spread over a much larger distance Clouds of dust between us and Betelgeuse absorb most of its light

A red giant is cool (about 3,500 K) but can be much more luminous than the Sun. How?

Red giants fuse helium instead of hydrogen, which releases far more energy per reaction Although it emits less energy per unit area because it is cooler, its enormous surface area more than compensates, making total energy output very high Red giants are actually hotter than blue stars — we just measure them incorrectly The luminosity of red giants comes from their thick outer atmosphere, not their core

The magnitude scale used by astronomers is unusual — lower numbers mean brighter stars. Which statement correctly describes this scale?

Higher numbers mean brighter stars — magnitude 6 is the brightest visible The magnitude scale measures colour — magnitude 1 is blue and magnitude 6 is red Each step of 1 magnitude is about 2.5 times brighter; a star of magnitude 1 is about 100 times brighter than a star of magnitude 6 Magnitude is a measure of a star's distance — magnitude 1 means 1 light-year away

What type of star has a surface temperature of about 10,000 Kelvin and appears blue-white?

A G-type star, like our Sun An M-type star, the most common in the galaxy An A-type star, such as Sirius or Vega A K-type star, slightly cooler than the Sun

Why do astronomers care about a star's color index — the difference in brightness measured through blue and visual (yellow) filters?

It helps identify which stars are visible to the naked eye The color index provides a precise, objective measurement of a star's surface temperature that avoids relying on subjective color descriptions It measures how much a star has dimmed over time due to aging Color index identifies which stars are in binary pairs