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astro > The Bizarre Stellar Graveyard > Flashcards

The Bizarre Stellar Graveyard Flashcards

1
Q

Degeneracy pressure supports a star’s remnant against

A

the crush of gravity

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2
Q

A degenerate star is supported by:

A

 Electron
(e–) degeneracy pressure→ white dwarf
OR
 Neutron degeneracy pressure→ neutron star (NS)
A massive remnant core, for which gravity > neutron
degeneracy pressure, can collapse out of existence
—–> black hole (BH).
 White dwarf = Exposed core of a dead (Sun-like) star
that has shed its outer layers in a planetary nebula.
 Progenitor star had insufficient mass to become hot enough to
fuse C
<50% of original mass left
 Gravity stopped by electron (e–) degeneracy pressure .

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3
Q

White Dwarfs

A 
Slide down the H-R
diagram as they radiate
their heat away, getting
cooler& fainter.
Very hot when formed→Hottest white dwarfs can radiate intense UV &X-ray light

No new energy generated→ the white dwarf eventually cools into a black dwarf
 Remember: the white dwarf does NOT shrink as it cools because e– degeneracy pressure is essentially independent of T !

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4
Q

The composition of a white dwarf depends on the mass

of its

A

 Very low mass→ He
~1MSun→ mainly CSize of white dwarfs
 Intermediate mass→ C with large amounts of O & heavier elements.

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5
Q

The average white dwarf has a mass near

A

0.6Msun

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6
Q

The average white dwarf has a mass near 0.5 … 1Msun compressed into

A

the size of earth

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7
Q

describe white dwarfs part 1

A 
Very dense→ A teaspoon of its material
weighs ~10 tons (as much as a truck)!
 They can have various ‘compositions’:
typically a C/O core with/without He
and/or H2 layers, which can also be
neutral or ionized.
 If existent, any of the remnant H & He layers must be quite thin to avoid
fusion at the base of the layer
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8
Q

describe white dwarfs part 12

A

Degenerate matter obeys different laws of physics.
The larger the mass, the smaller it becomes!
 Larger mass -> ↑ gravity -> ↑ density → smaller size -> e– degeneracy pressure must also ↑ to balance gravity
 A 1.3 MSun white dwarf is half the diameter of one with 1 MSun , which is about the size of Earth
 For white dwarfs the relation between mass & size is:
R proportional to M^(-1/3)

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9
Q

Heisenberg’s Uncertainty Principle sets a limit on

A

how massive a white dwarf can be: There is a maximum mass that creates a maximum e degeneracy pressure for which the e– reach the speed of light
 Anything above this mass requires e– to travel faster than the light speed !—>impossible!
 This is the Chandrasekhar Limit: no white dwarf can have a mass greater than this critical value
 This limit depends slightly on the composition of the
material; for a C/O core the Chandrasekhar mass is
Mcritical =1.4MSun

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10
Q

Chandrasekhar predicted that

A

gravity will overcome e – degeneracy pressure if the white dwarf mass exceeds a certain critical value Mcritical

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11
Q

fangs

A

Sometimes white dwarfs can be energy-sucking
vampires!
They can gain mass if companion is a main-sequence
or giant star.
 Falling matter forms an accretion disk
 Friction in disk — . :>visible, UV & X-ray light
 Differs from a protostellar disk in size (much smaller, due to stronger gravity from the white dwarf), orbital speed (much higher!) & T (much hotter!)

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12
Q

H build-up on white dwarf

surface—>

A 
p&T of this thin
layer of H ↑↑------> When
bottom of the layer reaches
T = 10mK------> Explosive
fusion reaction is
ignited! = NOVA!
 A gas shell is blown off
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13
Q

The white dwarf temporarily becomes brighter

—-> nova, please explain this

A

 Minor detonation of H fusion on white dwarf’s surface
 As bright as 100,000 Suns for a few days, then fade slowly
 The brighter the nova, the shorter its duration.
 Accretion may resume after explosion & process can repeat with a period of months to ~100,000 years (typically tens of kyears)
 The larger the white dwarf mass, the shorter the period.
 If enough mass is gained (1.4
MSun limit is reached!) —-> Supernova

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14
Q

An accreting white dwarf can

A

gain mass in time

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15
Q

Electron degeneracy pressure can no longer support star when

A

the accreting white dwarf reaches a mass equal to
or above the Chandrasekhar limit —>  White dwarf collapses!–>
↑↑↑ core temperature–> Runaway C fusion!
(C undergoes fusion all at once) —– > Massive explosion

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16
Q

white dwarft supernova

A

type 1a supernova
Nothing is left behind !
 In <1% of the cases, it is believed that a white dwarf may NOT explode as a Type Ia supernova but instead undergoes a process known as an accretion-induced collapse to form a rapidly rotating neutron star. However, NO accretion-induced collapse event has yet been directly observed experimentally

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17
Q

White dwarf supernovae are different from those

A

generated by Fe catastrophe (massive star collapse)

:type II supernovae

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18
Q

How are supernovae classified?

A

 Based on the presence or absence of certain elements → from the spectra of their maximum emitted light
 From the explosion’s luminosity: its peak value & variation in time

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19
Q

Hypernova

A

is an extremely luminous supernovae:
a stellar explosion with an energy of over 100 supernovae ,when the star’s core collapses → only for very massive stars, typically with Mstar > ~40MSun

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20
Q

shine most brilliantly

A

Type Ia supernovae shine most brilliantly: 10b Suns.
 Can temporarily
be as bright as a
whole galaxy!

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21
Q

Supernovae types distinguished by their

A

luminosity & its

variation in time

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22
Q

White dwarf supernovae (Type Ia) lack

A

H absorption lines & after a short high peak their luminosity fades steadily with time

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23
Q

 Supernovae of massive stars fade in

A

in 2 or more distinct stages

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24
Q

White dwarf supernovae always occur when the

A

1.4MSun limit is reached —> same maximum luminosities!
 Excellent distance indicators
 More luminous than Cepheid variable stars
 Can measure much greater distances

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25
Q

No mass limit for

A

massive star supernovae.

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26
Q

Distance measurement with white dwarf supernovae

A

 White dwarf supernovae always occur when the 1.4
MSun limit is reached—>same maximum luminosities!
 Excellent distance indicators
 More luminous than Cepheid variable stars
 Can measure much greater distances

No mass limit for massive star supernovae.

Knowing the energy (because
M is always the same, Mcritical = 1.4MSun!) of a white dwarf supernova–>Luminosity of ANY such Type Ia event is known–>Distance to the white dwarf supernova (& its galaxy) can be determined using the inverse square law:
Distance measurement with white dwarf supernovae
Apparent
brightness = Luminosity/4πd^2

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27
Q

Neutron star (NS) =

A

leftover core from the Fe core

collapse of a massive star’s supernova.

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28
Q

 Neutron degeneracy pressure stops

A

collapse of cores with masses <3MSun .

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29
Q

Gravity crush cannot be stopped for Mcore>3MSun

—->

A

black hole

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30
Q

Neutron Star  Tremendous crush of gravity:

A

 The gravitational acceleration at surface = 1011g (i.e. 100gb)
Escape velocity is ~0.5c!

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31
Q

 Neutron stars are very dense:

A

Mass ave≅ 1.3…1.5MSun with
diameter of ~10…20 km
ρ=1012 g/cm3→ denser than the atomic nucleus! A paperclip of this density outweighs Mount Everest!
 Densest matter known!!!
 In very dense matter, neutrons are stable

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32
Q

What is it’s a neutron’s star structure?

A

 ~90% is made of neutrons

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33
Q

The state of matter varies with depth inside a neutron
star
:

A

 Surface: a thin solid crust of &
e– & + nuclei
 Deep inside: A heavy liquid interior in which immense pressure
disintegrates atomic nuclei & recombines
e– with protons —> neutrons (hence the name). Various superfluids, e.g. superfluid of free neutrons & a superconducting fluid of protons are probably found at different depths

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34
Q

Neutron stars are also very hot:

A

T = 1mK≅ 170TSun (almost perfect blackbody radiators!)
 Hence, applying Stefan-Boltzmann’s law ->L = 0.3LSun
 Applying Wien’s displacement law-> λpeak
≅3 nm !, i.e. almost all light is emitted as X-rays
 Hence, we can find neutron stars in the Universe by observing the X-ray range. Still, some can be observed in visible light, and some (pulsars) with radio telescopes

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35
Q

Neutron stars cool down by 2 possible key mechanisms

A

 Also EM radiation, but most importantly: by neutrino emission
 In pulsars, also by radiation emission jets → also carry away angular momentum
 gradual slow down of the spin (spin-down rate ∝ mgn. field strength)

Despite their density and age, they still have a very
agitated life: 
Quakes → violent crust dynamics (cracked by the mgn. field??)
 Spin variations that shake the interior → confirm the interior is fluid
 Rapid & drastic mgn. field rearrangements —> GRBs in magnetars

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36
Q

Pulsars discovery

A

Graduate student Jocelyn Bell & her advisor discovered a
radio source in the Cygnus constellation in 1967.
 Observed a sharp pulse recurring every 1.33 s
 300 pc (978 light years) away
They called it a pulsar
(pulsating radio source).
Pulses came at very precise intervals.
 Man-made? Artificial??
Mystery solved by late 1968 when a pulsar was
discovered in the heart of Crab Nebula
Pulsations: due to rapidly spinning neutron star.

37
Q

Pulsars

A

The magnetic field at the surface of a collapsing star
grows in strength as the surface area of the star
decreases (↓ in radius leads to↑ mgn. field strength ~ 1/10^5) —> Huge magnetic field strengths at
the surfaces of neutron stars (even larger for
magnetars!!)

38
Q

Magnetar =

A

another type of neutron star: with an ultra-strong magnetic field

39
Q

Mgn. field of a neutron star:

A

1…10t× Earth’s

40
Q

Pulsars Rotate rapidly with periods of

A

0.03…4 s. Due to angular momentum conserv. during & after collapse

41
Q

potent electrical generator

A

The strong mgn. field & rapid rotation of a neutron

star make it a very potent electrical generator.

42
Q

Pulsars continue s’il vous plait

A

 The electric field generated by the rotating magnetized neutron star can be strong enough to rip charged particles (such as e–) away from its surface——->
The charged particles follow the mgn. field lines to the N & S mgn. poles of the neutron star and are accelerated —> Produce (not yet known how exactly) intense but narrow beams of radiation, pointing away from the two mgn. poles ————>
Radio emission
concentrated at mgn.
poles of neutron star
& focused into a beam

43
Q

Pulsars are

A

Neutron Stars!

44
Q

Magnetic poles usually do NOT coincide

with

A

rotational poles

45
Q

“lighthouse” effect

A

Beam of radiation sweeps round &
round when poles are not aligned with
rotation axis:

46
Q

The star’s orientation determines

whether we ‘see’ it as a pulsar:

A 
 Pulsar: We see a pulse of light/radiation
each time the beam sweeps by Earth’s
direction once each rotation
 No pulsar if polar beam always points
away from Earth
Pulsar≡ neutron star.
 Neutron star != Pulsar.
47
Q

The youngest pulsars have the

A

shortest periods
(thought to have resulted from supernova explosion).
Rotate as fast as 625 times per second

48
Q

Rotation rate gradually slows as neutron star ages.

A

Energy & angular momentum carried away by EM fields created from the continual twirling of magnetic field
 Crab nebula pulsar currently rotates at ~30 Hz → rotates less than half as fast 2,000 years later after its birth

49
Q

several ‘ms pulsars’ are observed in globular

clusters → very old

A 
 These must have spun-up due
to in-falling matter from a
companion star
 Matter infalling onto the
neutron star adds angular
momentum (spin) to it
50
Q

Neutron stars in close binary systems can also

steal matter from companion

A 
----------->accretion disk
 Similar to white dwarf binaries
 Mass transfer speeds up the (pulsar) rotation
 Can become as high as ~1,000 Hz
 Known as ‘millisecond pulsars’
51
Q

Neutron stars gravitation is much stronger than

that of white dwarfs

A

In-falling matter releases enormous energy → Accretion disk becomes hot enough to emit X-rays
Close binaries containing neutron
stars are known as X-ray binaries
(XRBins).

52
Q

X-ray bursters

A

H gas accretes onto the surface of the neutron star :
 A 1 m thick H shell forms on the neutron star
 Pressure at the bottom of this
layer is high enough for H to
fuse steadily on the surface
 A He layer forms underneath
 Its T steadily ↑ —> when it reaches 100m K, He fuses instantly & emits a huge energy burst
Neutron star “novae” are called X-ray bursters.
 A burst lasts a few seconds & repeats every few hours to days
 Each burst radiates as much as 100,000 Suns!!

53
Q

Anomalous X-ray Pulsars (AXPs)→

A

→ A different kind of X-ray emitters, solely due to events on the surface of a magnetar

54
Q

Double neutron star system

A

Many neutron star (NS) pairs are now known.
These stars spiral toward each other in an orbital decay, emitting space-warping gravity waves.
 Similar to light waves radiating outward like ripples on a pond
Eventually collide & merge, possibly becoming a black hole (BH)
Such pairs have given us the first (indirect) evidence of the existence of gravitational waves
 The radioactive material released in the merger heats up and expands, emitting a burst of light called a
kilonova as well as a slew of γ rays in a GRB. While kilonovas are 1,000 times brighter than a nova, they’re 1/10th to 1/100th the brightness of a typical supernova.
 kilonova = NS+NS or NS+BH merger

55
Q

Double neutron star system 2

A

How can very heavy elements be produced by stars?
 Unlike elements like C, Si or Fe, Au or U cannot be created within the core of a star.
Kilonovas = NS+NS and NS+BH mergers create
significant quantities of neutron-rich radioactive
nuclei via the so-called r-process = the rapid neutron capture process
When the spiralling NSs finally merge, the matter
reaches unimaginable temperatures (1011 K). A few
percent of the matter is ejected in the form of spiral arms, which cool rapidly. It is in these arms that the important
nuclear physics takes place which creates elements
heavier than Fe, a list that includes Au, Hg, Pt, U, Th, and more.

56
Q

Gravity’s ultimate victory: Black holes

A

The core of a massive star with
Mcore>3 MSun after supernova will collapse without end.
 Gravity too strong for even neutron degeneracy to stop the collapse
 Gravity final wins! ——-> The star becomes infinitely small ——-> Creates a “hole” in the universe
This is what we call a black hole

57
Q

This is what we call a black hole

A

huge mass compressed into an infinitely small space has extremely large gravity!—> Black hole = an object so dense that escape velocity exceeds c
 A singularity!
 Newton’s law of gravity no longer valid!

58
Q

According to the Theory of
Relativity, gravity is not a
force but the

A

warping of spacetime (the 4D Universe) about
an object with mass ——-»»>Gravity imposes a curvature on the space-time ‘fabric’.
 Space-time extremely distorted near a black hole
 Path of light through space is bent
Even light (no mass) is affected by gravity.

59
Q

event horizon

A

Once one enters a black hole, (s)he leaves the
observable universe & can never return.
Matter/light cannot climb out once within a
boundary known as the event horizon:
the boundary around a black hole at
which the escape velocity equals the speed of
light (c)
.
 Tidal forces could be tremendous near event horizon
 The entering object could be “spaghettified”

60
Q

event horizon 2

A

Boundary between the inside of black hole & the Universe → defined by a sphere of radius RS around the black hole
 Inside of black hole = the actual singularity but also its close vicinity
 Point of no return for objects entering black hole
 Also defines the size of black hole

61
Q

The Schwarzchild radius,

RS

A

RS = 3M (Rs in [km] if M is in [MSun])
 Black hole with the mass of the Sun: RS = 3 km
 Black hole with 10MSun: RS = 30 km

62
Q

Stellar core within event horizon elaborate

A

Hidden from view
 Contains all the mass & exerts full
gravity of the mass

63
Q

All matter crushed to an infinitely tiny & dense point

A

singularity

64
Q

Very far away from a black hole, space-time is less
curved and its gravity is indistinguishable from
ANY other mass

A

A black hole at a distance exerts gravity according to Newton’s Law:
 Planets’ orbits the same if a 1MSun black hole replace our Sun
 It does not suck in everything around it!
Gravity increases from Newton’s predictions only at a 3RS distance from the black hole

65
Q

Properties of a black hole (continued)

A

The black hole destroys nearly all information about
matter that falls into it—->We CANNOT answer the question “What lies inside a
black hole?”→ because no information can ever
emerge from within the event horizon
Conservation of angular momentum dictates that black holes should rotate very rapidly when they form in the collapse of a rotating star–>It will drag neighbouring regions of space-time around it in circles
 Effect known as frame-dragging

66
Q

Frame-dragging will tend to

A

accelerate infalling objects in

the direction of rotation, instead of falling straight forward —– > Fast spinning accretion disk

67
Q

Friction between adjacent zones of the accretion disk causes it to become

A

extremely hot and emit large amounts of X-rays

68
Q

Friction between adjacent zones of the accretion disk
causes it to become extremely hot and emit large
amounts of X-rays

A

This heating is extremely efficient: can convert 10…50% of the mass energy of an object into radiation, as opposed to nuclear fusion which can only convert a few % of the mass to energy
 Other predicted effects are narrow jets of particles at relativistic speeds squirting off along the disk’s axis

69
Q

Light near a black hole

A

Light is trapped due to effects of general relativity:
space-time is severely warped within the event horizon.
Light coming from a region close to a black hole is bent (if not radial) & red-shifted.
Light can orbit a black hole at a radius of 1.5RS

70
Q

Falling in a black hole

A 
Time slows down near a
black hole.
What would it be like to
plunge into a black
hole
71
Q

What would it be like to
plunge into a black
hole 1

A

 An astronaut takes a probe to go into the black hole
 From the astronaut’s point of view, time neither speeds up nor slows down, but he’d say that the mother-ship time runs increasingly fast & its light gets blue-shifted

From the mother-ship’s point of view, the probe’s on-board clock ticks more slowly & light from probe is red-shifted → because the photon must spend energy to overcome the hole’s gravity. This energy can come only by “giving up” of the initial energy by increasing its wavelength

72
Q

What would it be like to
plunge into a black
hole 2

A

Probe eventually disappears from view as light is red-shifted beyond radio & time on clock stops
 From the mother-ship point of view, the astronaut never crosses the event horizon, will asymptotically be approaching it FOREVER.

If the black hole is small (e.g. 10
MSun , with RS= 30 km) —> the
astronaut is “spaghettified” & torn apart by tidal forces

 If the black hole is supermassive (e.g. 10^9MSun , with RS= 3bkm) —–>the tidal forces are much smaller & non-lethal

73
Q

A black hole by itself emits no light

A

difficult to detect

74
Q

A black hole is the only object that can be

A

so massive & yet small enough to be invisible → 3 main ways to detect them

75
Q

A black hole is the only object that can be so massive & yet small enough to be invisible → 3 main ways to detect them

A

 1) Infer its presence from the effect exerted on its
companion in X-ray binaries which contain a black
hole.
 2) Gravitational lensing

 3) Observe sources of strong radiation (γ, X-ray) &
emission jets.

76
Q

Gravitational lensing

A

Gravitation of any huge mass curves space so that light travels on a curved path—> the mass acts as a lens
 Another effect: the star being lensed by a foreground mass gets brighter when the mass passes in front of the star.
 It is possible to measure the mass of the “lens”

77
Q

Observe sources of strong radiation (γ, X-ray) &

emission jets.

A

 Now we know that almost any galaxy must have a huge black hole at its center (usually supermassive, 10^6MSun at center of Milky Way)
 The ‘light signatures’ (particularly the X-ray spectrum) from the
accretion disk & radiation jet emitted by a BH are different from
those coming from a NS and also the intensity variation in time
 Still, it is not always possible to tell with certainty what object is at the center of the accretion disk

78
Q

Just like white dwarfs & neutron stars, a black hole can also be

A

one component of a binary system

79
Q

Just like white dwarfs & neutron stars, a black hole can also be one component of a binary system

A

an accretion disk will form & emit hard radiation (as shown earlier)

80
Q

But is there any binary system formed ONLY of black

holes?

A

When 2 galaxies merge with one another —> was experimentally confirmed recently
 Astronomical binaries of massive stars might also lead to the formation of binary black holes.

81
Q

Black holes can also

A

merge or collide
 Huge amount of radiation emitted as gravitational waves during the final second before the two black holes merge: could be 5% of the total mass of the system —> more energy than a star of comparable mass emits in optical radiation throughout its billions of years of life-time.
 Black hole binaries could well be Nature’s most luminous objects!

82
Q

Gamma Ray Bursts (GRBs)

A

The most powerful bursts of energy that ever occur in
the universe
 Total luminosity of a burst can briefly exceed combined
luminosities of 1m galaxies like our Milky Way!

83
Q

Cosmic γ-rays observed above Earth’s atmosphere since 1960s.

A

 Satellites detected strong bursts, which occurred only for a few minutes or less, but almost daily
 Hard to focus, pass through telescopes
 Unable to determine direction

84
Q

Afterglows of some GRBs detected at other wavelengths since 1997.

A

 Pinpoint their sources to distant galaxies

 Some can even be seen with binoculars!

85
Q

Two types of GRBs:

A

 Short (duration <2 s)

 Long (duration >2 s)

86
Q

grb explain more pls: Short bursts tend to be

A

10 × dimmer and have more highly-energetic (hard)

γ rays than long ones.

87
Q

Short bursts

A

the energy conversion of the source/phenomenon into γ rays ↓ as the burst progresses, unlike long bursts where it appears to remain constant throughout the burst

88
Q

what causes short ray grbs

A

Long bursts → Supernovae or even hypernovae = gigantic supernovae of very massive stars (rapidly rotating core) forming black holes
.
Short bursts → thought to be due to catastrophic
collisions of 2 very massive stellar objects in a binary
system ( 2 neutron stars, or neutron star & black hole).

astro (14 decks)

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