ASTRONOMY TEST #5

Stars form in dark clouds of

gas and dust.

The clouds that form stars are typically pretty could, like

10-30 K.

The clouds that form stars typically have a mass of

at least a few hundred solar masses. These large clouds contain enough material to collapse under their own gravity, leading to star formation.

Protostars are

the places in the clouds where stars form.

How long did it take for the sun to form?

The whole process of collapse of the cloud to formation of a star takes about 30 million years for the Sun, less time for more massive stars.

About ___ the stars in the universe are in a binary system.

Half.

___ mass stars are much more commons than ___ mass stars.

Low; high.

What are the upper and lower mass limits for stars?

Lower limit is about 0.08 solar masses, and the upper limit is around 150 solar masses.

What is the minimum temperature required for fusion reactions?

10 million K.

What is a brown dwarf?

A protostar that stops collapsing before it reaches the minimum temperature to become a star.

Why are brown dwarves hard to detect?

They are very common but emit barely any light.

What is degeneracy pressure?

Degeneracy pressure is a quantum mechanical effect that occurs when particles are packed closely together, preventing further compression. It is significant in supporting white dwarfs and neutron stars against gravitational collapse.

What are low mass stars?

Stars that have less than twice the mass of the Sun.

What are high mass stars?

Stars that have more than twice the Sun’s mass.

What are main sequence stars?

Stars that fuse hydrogen and helium together in their cores. They are in a stable phase of stellar evolution.

How long do stars stay on main sequence?

90% of their lives.

Where do giant stars come from?

These stars originate from main sequence stars that have exhausted the hydrogen in their cores and have begun to fuse heavier elements, leading to expansion and cooling. They live 10% of their lives as these types of stars.

How does helium fusion work?

When this begins it’s called the “helium flash.” When this happens, the temperature goes way up, & the thermostat is restored. The star shrinks from giant size.

How does helium fusion work?

Eventually the temperature in the core goes up to 100 million K. This is high enough for helium to fuse into carbon. It takes three helium atoms to make one carbon atom.

What are double-shell fusion giants?

Double-shell fusion giants are stars that fuse hydrogen in a shell surrounding an inert helium core while simultaneously fusing helium in an outer shell. This process leads to significant expansion and multiple layers of fusion activity.

What is a planetary nebulae?

A process that expels a pulse that ejects the remaining H & He into space.

What is a white dwarf?

The carbon core left behind. They are typically the size of the earth. They are formed when a low-mass star has exhausted all its central nuclear fuel and lost its outer layers as a planetary nebula.

How are the lives of low-mass stars different from the lives of high-mass stars?

Every stage of a high-mass star’s life proceeds faster than for a low-mass star. The stages are the same, except at the end when the star gets to carbon.

What is multiple-shell burning?

A process occurring in high-mass stars where nuclear fusion occurs in multiple layers, enabling the burning of heavier elements in distinct shells as the star evolves. High-mass stars have enough gravity to cause carbon to heat up enough for additional nuclear reactions to happen. Various reactions occur in different layers of the star.

What are helium capture reactions?

Nuclear fusion processes in which helium nuclei combine with heavier elements, producing even heavier elements during stellar evolution.

Why is iron a “dead-end”?

Nuclear reactions can continue ‘till you get to iron, because fusion reactions involving heavier elements don’t release energy, they consume it.

How are “heavy” elements produced?

The result of multiple-shell burning is that a variety of heavy elements are produced. The more massive the star, the more heavy elements are produced.

Why are even-numbered elements more common than odd-numbered ones?

The reason is that they result from helium-capture reactions, & near the end of a star’s life there’s lots of helium available.

What is a massive star supernova?

An explosion called a ________ occurs when the core collapse happens; this explosion blows a lot of the material off into space.

When did the crab nebula supernova occur?

The supernova occurred in 1054 A.D., and was seen by people all over the world.

What can happen in close binaries?

When one star in a binary system is much more massive then the other, unusual things can happen. An example is Algol, which has a main-sequence star 3.7 times the mass of the Sun, and a subgiant star 0.8 times the mass of the Sun.

The properties of white dwarves are

related to their mass and composition, influencing their temperature and brightness as they cool over time. 1% the size of the star it came from (about the size of earth). They start out hot but cool and reden over time. The higher its mass is, the smaller it is.

The properties of neutron stars are

are balls of neutrons left over after the explosion of a massive-star supernova. Remember that electron degeneracy pressure isn’t strong enough to prevent a massive-star from collapsing, and that in this case electrons & protons combine to form neutrons & neutrinos. The neutrinos escape into space leaving only neutrons. The degeneracy pressure of the neutrons is strong enough to prevent further gravitational collapse---to a point. 

The properties of black holes are

If the mass of a dying star exceeds about 3 times the mass of the Sun, the gravity becomes so strong that it overwhelms the neutron degeneracy pressure. Then what was a neutron star collapses down to a point called a singularity. When this happens you have a black hole. But it does have an event horizon. This is the closest distance you can get from it & still have some possibility of coming back. Once you get beyond the event horizon, then there’s no possibility of coming back---not even light can come back. The event horizon for a black hole 3 times the mass of the Sun is about the size of a small city---not too much different from the size of a neutron star.

What can happen when a white dwarf is a part of a binary system?

We saw stars can exchange mass as parts of binary systems.

When a white dwarf is involved, the mass forms a flat accretion disk as it spirals in to the white dwarf due to angular momentum conservation.

Friction in this disk heats it up & makes it glow.

Massive-star supernovas have an

iron core of massive star reaches white-dwarf limit, collapsing into a neutron star & blowing some---but not all---matter out into space.

White-dwarf supernovas

carbon fusion begins as a white dwarf nova repeatedly goes of & the leftover helium reaches the white-dwarf limit. A total explosion occurs, & nothing is left over.

How do you tell the difference between a nova and a supernova?

A supernova is about 10 million times as bright as a nova.

When a nova occurs, there is an explosion, but the white dwarf remains intact.

When there’s a white-dwarf supernova, the explosion is so large that nothing is left behind.

What are pulsars?

LGM is an example of what became called pulsars; there were soon found to be more of them at different places in the sky. In the end, research showed that they were rotating neutron stars which emitted these beams of radiation. The had to be neutron stars because nothing larger could rotate so fast without tearing itself apart. Further measurement showed that the pulses weren’t is precise as was originally thought, but gradually change over many years as the neutron star rotates slower & slower.

What is the event horizon?

Event horizon, boundary marking the limits of a black hole.

Where do supermassive black holes usually exist?

In the center of galaxies.

What is Cygnus X-1?

A binary star system that is one of the strongest X-ray sources in the sky and is believed to contain a black hole.

What are gamma ray bursts?

Gamma-ray bursts (GRBs) are extremely energetic explosions observed in distant galaxies, often associated with the collapse of massive stars or mergers of compact objects.

What causes gamma ray bursts?

Gamma ray bursts are caused by explosive events, such as the collapse of massive stars or the merger of neutron stars, resulting in the release of immense energy and gamma radiation.