ASTRONOMY U5 - Lifecycle of Stars

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Morgan Keenan System

Most up to date system. O0 is the hottest star, M9 is the coolest.

Color convention

Most stars appear white when so far away, or red due to red-shift.

Binary star systems

• Generally stable

• Two stars orbiting each other around a barycenter

  • Barycenter: outward center of mass

• Looks like a single object

• Brighter star is primary, other is secondary

• Long orbital periods (centuries, millennium)

• Multiple systems = 3+

• Found by one star passing in front of the other, creating a large dip in luminosity

Stellar nucleosynthesis

Nuclear fusion - to make a nucleus in a star

• Low mass main-sequence stars use proton-proton chains**

• High mass main-sequence use CNO cycles

Helium fusion

Main sequence stars don’t use their helium so it gets stored. Once they run out of hydrogen they start fusing helium

Protostars

• first star step

• forming

• multiple together are called a nursery

Stellar Life Cycle

• all stars start on the main sequence and then eventually change

Massive Star

• more mass than 8M

• H fusion

• Main sequence

• Shorter lifespan

Red Supergiant

• spend most of time in old age

• largest stars in universe’ in terms of volume

  • not most mass / luminosity

• cool and large

• <4100K

• typically 10M and 40M mass

• can produce observable nebulae surrounding the star (shed layers like onion)

• more massive ones lose mass faster

• rare but visible at great distances

• old

Supernova II

• “type two ___”

• caused by rapid collapse and violent explosion of a massive star

• mass between 8-50M

• hydrogen core / spectra is still present

• typically found in the spiral arms of galaxies, but not the elliptical

• leaves behind remnants

• death

Supernova Remnant

• remains / long-term of a star after a supernova

• made up of gases and elements that were created during fusion

• major source of cosmic rays

• death

Neutron Stars

• collapsed core of supermassive stars between 10 and 25M that aren’t big enough to form black holes

• smallest and densest known stellar objects

• radius of 6 miles and mass of 1.4M

• composed almost entirely of neutrons

  • electrons and protons combine into more neutrons

• high surface temperature (600,000K)

• extremely strong magnetic field

• corpse

Pulsar

• Pulsating radio source (like a lighthouse / radio)

• Type of neutron star that emits beams of electromagnetic radiation out of its magnetic poles

• Galactic lighthouse

• Big magnetic field, like from Sun to Pluto

• Corpse

Stellar Black Hole

• Anything above 40M mass

• Gravity is so strong it affects light

• Explodes inwards due to gravity

• Condenses all the mass down to a singularity

• Can only be detected due to bending of electromagnetic radiation around it

• The boundary of no escape is the event horizon (like a tractor beam), where spaghettification occurs

• Forms from the collapse of massive stars

• Can continue to grow from absorbing mass

Supermassive Black Hole

• made up of millions of M mass since it ate up other black holes and stars

• one is thought to be found at the center of most galaxies

• can form quasars - mass falling into the black hole heats up and releases energy, going both ways

• stars can orbit these

White Hole

• theoretical

• also called wormholes or Einstein rosenbridge

• region of spacetime and singularity that can’t be entered, though energy-matter and light can escape

• opposite of a black hole

• argued to be what the Big Bang was

Brown Dwarf

• Proto-star with less mass than 0.08M

• “Failed star” - can’t form fusion

• Not high enough temperature

• Shine dimly and fade away slowly

• Cooling gradually over hundreds of millions of years until eventual crystallization

Low Mass Star

• Proto-star with less mass than 8M

• H fusion first and then He fusion

• Main sequence star

• Has the longest lifespan of all stars

• The Sun is this!

Red Giant

• Two types

  • Asymptotic-giant-branch

    • 0.1-06M, nothing happening in core (inert) made of C and He inside H

  • Red-giant-branch

    • (0.6-10M), nothing happening in core (inert) made of He and H

  • The Sun is this!

Red Dwarf

• 0.1M, stays on main sequence for ~6-12 trillion years

• Live the longest

• Skip the explosion stage compared to red giants

• Eventually collapse into a white dwarf made entirely of He

Planetary Nebula

• Expanding, glowing shell of ionized gas

• Ejected from red giants late in life

• Formed from red giants around 0.8-8M

• Our Sun will do this!

• Short lived (tens of millennia)

• Ultraviolet radiation emitted, causing it to glow and expel elements into space

White Dwarf

• Stellar Core remnant composed of electron-degenerate matter

• Mass is equal to Sun but the volume of the Earth - dense

  • Mass not high enough to be black hole or neutron star

• Faint luminosity due to emission of residual thermal energy

• No fusion - not hot enough

• Mostly Carbon and Oxygen but other variants exist based on mass

  • Will eventually occur to the Sun

  • Small stars

  • Corpse

Black Dwarf

• Theoretical stellar remnant that emits no heat or light

• We have not existed long enough for white dwarfs to cool to this

• Crystallized

• Emits little to no radiation but still has gravitational influence

Binary White Dwarf

• Formed from a binary star system with one white dwarf in it

• White dwarf can keep itself going longer by siphoning energy away

• Companion stars influence the details of how they age

• Gravitational waves generated from these systems are extreme

Supernova type 1a

• Greater than 1.4M mass

• Occurs when a binary system has only one white dwarf

• White dwarf absorbs enough that it fails fusion and triggers runaway nuclear/thermal fusion

• Explodes powerfully and luminously

• No hydrogen

Nova

• Less than 1.4M mass

• Occurs with binary star system

• Can happen repeatedly if stars come in close contact again