Stars: Colors and Life Cycle

• Stars are giant, hot balls of gas, mostly hydrogen, with some helium and trace elements.
• A star's life cycle ranges from a few million to trillions of years, with properties evolving over time.

Colors of Stars

• Star color indicates temperature: red stars are coolest, followed by yellow, white, and blue stars which are the hottest.
• Brown dwarfs are "failed stars" lacking the mass for core nuclear fusion.
• Stars appear round like the Sun, but their distance affects brightness.
• Starlight bends and scatters in Earth's atmosphere, causing stars to twinkle.

Life Cycle of a Star

• Stars undergo birth, growth, life changes, and eventual death.

Nebula and Protostar Stage

• Nebulae, giant clouds of gas and dust, are stellar birthplaces.
• Some nebulae form from dying star explosions (supernovae), while others are "star nurseries".
• Gravity pulls gas (mainly hydrogen) into a spinning cloud, increasing atom collisions and generating heat.
• Nuclear fusion begins as the cloud heats, resulting in a glowing protostar.
• Protostars need to reach 27 million degrees Fahrenheit to complete this stage.
• As a protostar collapses, a hot, dense core forms, gathering gas and dust.
• Remaining dust may form planets, asteroids, or comets.

Main Sequence Star Stage

• Nuclear reactions start once a protostar is hot enough.
• Stars fuse hydrogen into helium in their cores, becoming main sequence stars.
• Star color depends on its temperature at this stage.
• Stars can remain in this stage for thousands to billions of years.
• Stars like the Sun take about 50 million years to mature from collapse to adulthood.

Giant Stage

• The next life cycle stage depends on the size of the main sequence star.
• Smaller stars' paths diverge from larger ones at this point.
• Smaller stars become red giants after billions of years, as nuclear reactions slow, cooling, reddening, and expanding the star.
• Larger stars become supergiants, growing a thousand times bigger than the Sun as nuclear reactions slow.

Cooling or Explosive Stage

• Energy and heat decrease, and stars lose their outer shells in the giant stage.
• Smaller stars that have fused all core hydrogen end nuclear reactions.
• Small stars live over 10 billion years, ejecting outer layers over 10,000 years, leaving a hot white dwarf core.
• White dwarf radiation causes ejected gas to glow, forming a planetary nebula.
• Massive stars become red supergiants and explode as supernovae.
• Supernovae leave behind neutron stars, black holes, or nothing at all.
• A supernova can briefly outshine its galaxy before fading, leaving a neutron star or black hole surrounded by hot gas.

Black Holes

• Black holes are areas where gravity is so strong that even light cannot escape.
• They aren't holes, but places where matter is squeezed into a small space, often during a star's death.
• Black holes are invisible, but space telescopes can find them.
• Smallest black holes may be atom-sized, but with a mountain's mass.
• Stellar black holes can have 20 times the Sun's mass.
• Supermassive black holes have over a million times the Sun's mass, found at galaxy centers.
• Sagittarius A, at the Milky Way's center, has 4 million solar masses.
• Black holes don't "eat" stars, planets or moons, and are not a threat to Earth and the solar system.
• The Sun isn't big enough to become a black hole.