ASTR 2010 Exam 2 (Week 10)

Main-sequence phase of a low-mass star

Will last around 10 billion years. Fusing hydrogen into helium in the core. Maintaining hydrostatic equilibrium.

Red giant phase of a low-mass star

Will last about 1 billion years. Fusing hydrogen into helium in a shell surrounding the core. Searching for a new equilibrium.

Yellow giant phase of a low-mass star

Will last about 100 million years. Fusing helium into carbon and oxygen in the core. Maintaining hydrostatic equilibrium again.

Yellow Giant phase ends in a low-mass star

Core runs out of helium (no more fuel), nuclear fusion stops, star falls out of hydrostatic equilibrium, gravity contracts core, core temperature rises.

Red giant phase (again) for a low-mass star

Shell fusion causes the outer layers to expand. The yellow giant becomes a red giant again. Brighter and larger than it was the first time as a red giant. Will only last about 10 million years.

During the second red giant stage the sun will become a…

Mira Variable star

Mira Variables

Pulsating stars in the second red giant phase for low-mass stars. The star expands and contracts in repeated pulsations.

Stella wind

An outflow of gas from the star (can be produced from Mira Variable Stars).

Planetary Nebula

A shell of gas ejected by a low-mass star late in its evolutionary lifetime.

White dwarfs

The remains of dead low-mass stars. Have a mass less than 1.4 solar masses, A radius similar to the Earth’s, and are very dense (the compression of the core condensed a high mass into a small space.

White dwarfs are…

Very hot

The light from a white dwarf does not come from fusion but instead comes from…

Residual heat leftover. The stellar remnant will slowly dim as it cools off over billions of years.

Mass transfer

Process by which one star in a binary system transfers matter onto the other.

Bright explosion from an exploding white dwarf

nova

Chandrasekhar limit

The maximum mass a white dwarf stellar remnant can have. This is approximately 1.4 solar masses.

If a white dwarf exceeds the chandrasekhar limit (1.4 solar masses) it will?

Collapse uncontrollably.

What happens in high density/high temperature environments?

Nuclear fusion

Type 1a Supernova are estimated to be one of the…

Primary sources of elements between silicon and zinc on the periodic table.

Type 1a supernova

An extremely energetic explosion produced by the abrupt fusion of carbon and oxygen in the interior of a collapsing white dwarf star.

supernova remnant

The matter from the exploding white dwarf becomes a cloud of hot gas.

The type 1a supernova serve as excellent…

standard candles (all have about the same luminosity).

Stages of evolution of low-mass stars in order

Protostar → pre main-sequence star → Main-sequence star → Red giant → Yellow giant → Second Red giant → Planetary Nebula → White Dwarf

During which of the stages may the low-mass star become a T Tauri variable star?

Pre main-sequence phase

During which of the stages may the low-mass star become an RR Lyrae variable?

Yellow Giant phase

During which of the stages may the low-mass star become a Mira Variable?

Second red giant phase

Low-mass stars

The evolutionary track that stars take if they have less than 8 solar masses at the start of the main-sequence phase.

Example of low-mass star?

The Sun

Where are high-mass stars located?

On the upper main sequence.

High mass stars have…

Shorter main-sequence lifetimes than low-mass stars.

Further fusion in high-mass stars cycle

Fusion creates a new element → Core fuel runs low → core contracts and heats up → new fusion reactions use heavier elements → Process repeats producing heavier elements.

What is the end state for the fusion occurring in high-mass stars?

iron core.

Core-collapse supernova

A star that begins with more than about 8 solar masses, will fuse elements all the way up to iron in its core, which then collapses catastrophically.

Supernova remnant

The debris ejected from a star when it explodes as a supernova.

The amount of heavier elements in the Interstellar medium (ISM) increases as…

more dying stars expel supernova remnant.

Nucleosynthesis

The formation of elements by fusion of lighter elements into heavier ones.

What were all the elements in the universe that are heavier than helium made by?

fusion in stars

The observed abundances of the elements in the universe

75% Hydrogen, 23% Helium, 1% Oxygen, and 0.5% Carbon

Any small disturbance can…

compress a molecular cloud.

A supernova causes…

A new star formation. The heavier elements created by the dying star go towards forming new star systems.

Neutron Star

The outer layers of a high-mass star are expelled in a supernova explosion. The collapsed core may survive the explosion and it is made of neutrons.

Neutron Stars

Small radius → small luminosity → large temperature. They fall outside the range we can see.

If the collapsing core is too massive (>20 solar masses) to be supported as a neutron star…

The core will continue to collapse until it forms a black hole.

Black Hole

An object whose gravitational attraction is so strong that its escape velocity exceeds the speed of light, preventing anything from leaving its “surface”.

Escape Velocity

The minimum speed an object must have to escape a body’s inward pull of gravity.

Schwarzschild radius

If a star is compressed down to this tiny radius, it becomes a black hole.( Rs = (2GM/C²) ).

What is the point called that all of the matter in the core of the star collapses down to one singular point (blackhole formation)?

Singularity

Event horizon

The invisible “boundary” around a black hole. The distance from the center at which the escape velocity is greater than the speed of light.

The Schwarzschild radius can also be thought of as the…

radius of the black hole

How do stars form?

From a collapsing molecular cloud.

Stars usually form in clusters explanation

A large molecular cloud may break up into thousands of separate “clumps”, each collapsing on its own. Each clump can form a new star.

All the stars within a given cluster…

Formed at the same time and formed from the same material.

Star cluster

A group of stars, held together by their mutual gravity.

Two main types of star clusters

Open clusters and Globular clusters.

Open clusters

A loose cluster of stars, held together by gravity. A few hundred stars scattered fairly loosely. Typically 7-20 light years across.

Globular clusters

A dense grouping of old stars. Could be up to millions of stars. Stars are more strongly bound together and are pulled into a dense ball. Typically 40-160 light years across.

H-R diagram of the cluster stars contains mostly red giants and white dwarfs →

Cluster must be old.

H-R diagram of the cluster stars contains mostly pre-main sequence stars →

Cluster must be young.

Very young clusters

Only protostars and main-sequence stars.

Young clusters

Mostly main-sequence stars and a few protostars or red giants.

Old clusters

Only main-sequence stars and red giants.

Very old clusters

Mostly red giants and a few main-sequence stars and yellow giants.

Main-sequence turnoff point

The point at which stars leave the main sequence and become a red giant.

Ranking cluster ages based on the main-sequence turnoff point

The lower the main-sequence turnoff point, the older the star cluster.

The Milky Way

The galaxy to which the sun belongs. Observed as a bright, concentrated band of stars across the night sky.

Galaxy

A system of stars, gas, and dark matter held together by gravity (the milky way is one example).

Shape of the Milky Way

Taken from studies of other galaxies, our galaxy is a flat disk with spiral arms.

3 parts of the Milky Way

Disk, Halo, and bulge.

Disk

The flat, round portion of a galaxy (the sun lies here in this part of the Milky Way).

Halo

The spherical region surrounding spiral galaxies. Contains all the globular clusters in our galaxy.

Bulge

The dense, central region of a spiral galaxy.

RR Lyrae Variables

All have about the same luminosity (about 40 solar luminosities). They are standard candles.

Harlow Shapley

Made a map of the globular clusters in the galaxy.

Interstellar dust

Blocks out light and makes things dimmer than they actually are.