Stellar Evolution: From Molecular Clouds to White Dwarfs

Stellar Birth and Molecular Clouds

Stars are born deep in molecular clouds—cold (10–30 K) dense nebulae. These regions are so cold that H₂ can exist. A cold cloud can fragment when gravity overcomes thermal pressure in dense regions; these regions (cores) become more dense and compact. A molecular cloud fragment heats up as gravity makes it contract, producing a protostar at its center. Conservation of angular momentum ensures that protostars rotate rapidly and are surrounded by spinning disks of gas.

Protostars and Stellar Mass

Stars more massive than about 100 M_sun blow off their outer layers, while protostars smaller than 0.08 M_sun become brown dwarfs that never get hot enough for efficient hydrogen fusion.

Main-Sequence and High-Mass Stars

A main-sequence star’s mass determines both its luminosity and its surface temperature. Contrary to common misconceptions, more massive stars live much shorter lives because they fuse hydrogen at a much greater rate. A high-mass star lives a short life, rapidly fusing its core hydrogen into helium via the CNO cycle. All low-to-intermediate-mass stars follow life stages similar to those of our Sun, while high-mass stars live short but brilliant lives and die in supernova explosions.

Giants and White Dwarfs

Giants and supergiants are stars that are nearing the ends of their lives. White dwarfs are the cooling embers of stars that have exhausted their nuclear fusion fuel. They are stars located near the lower left of the Hertzsprung-Russell diagram that are small in radius and appear white in color because of their high temperatures.

Characteristics of White Dwarfs

• Stability: They are stable due to the balance between gravity and electron degeneracy pressure.
• Energy: They generate no new energy. As they radiate their heat into space, they get cooler and fainter.
• Density: They are extremely dense, with 0.5–1.4 M_sun packed into a sphere the size of the Earth. This is 1 million times as dense as water and features 100,000 times Earth’s surface gravity.

Binary Systems and Novae

If a white dwarf is in a close binary system:

• Matter from its companion can be accreted onto the white dwarf.
• The matter forms an accretion disk—a rotating disk of gas orbiting the star formed by matter falling onto it.
• Friction in the accretion disk heats it, causing it to emit visible, UV, and even X-ray light.
• If matter falls onto the white dwarf, hydrogen fusion begins, and the white dwarf temporarily gets brighter.

These events are known as novae. They typically increase in brightness considerably for a few days, then fade.