Transcript Document

Black Holes
If the mass of the core is more than about
3 solar masses the neutron degeneracy is
overwhelmed and the core goes on collapsing.
When the core diameter reaches the
Schwarzschild radius (9 km for a 3 solar mass
remnant) nothing, not even light, can escape.
It becomes a “black hole”.
We can’t see a black hole directly but we can
see X-rays coming from hot material which is
falling into it.
The binary system Cygnus X-1 appears to
contain a black hole which is sucking material
in from its blue supergiant companion star.
Figs. Z17.36 & K15-21 or 15-23
PHYS1142
39
Interstellar Matter - ISM
Gas: mostly hydrogen and helium but heavier
elements present. Exists as molecules, atoms
or ions depending on temperature (lecture 2).
Nebulae: e.g. the Orion Nebula - gas heated
by ultraviolet light from young stars, shows
emission lines. Can also find absorption lines
due to cold interstellar gas clouds showing
sodium, potassium, calcium, iron etc.
Molecules: infrared spectra show presence of:
H2 , carbon monoxide, H2O, ammonia,
methane, methanol, ethanol, NH2CH2COOH
an amino acid - a building block for proteins.
Exist in cool (10 K), dark, clouds of about a
million million molecules per cubic metre (as
compared to ten million million million
million in a lecture theatre).
PHYS1142
40
Interstellar Matter - ISM
DUST:
1 grain per cubic football pitch
Temperature about 100 K; emits in infrared
Grain size about a millionth of a metre (1mm)
Core: silicates, iron, graphite
Mantle: icy solids of carbon dioxide, water,
methane, ammonia
Surface layer: large molecules, tars etc.
Sticky surface of dust grains may help
molecules to form.
Dust obscures the centre of nebulae photograph using infrared light which
suffers less scattering by the dust
(wavelength of infrared light longer
than size of dust grains).
Figs. Z15.15 &
K13-12
PHYS1142
41
Recipe for a Planetary System
Take 10,000 solar masses of molecular
gas chilled to 10 K.
Sprinkle liberally with carbon and
silicate dust seasoned with metals. Stir
well. Hammer to a lumpy consistency.
Stars will form and bake.
In other words...
Gas and dust cloud is compressed by
shock wave from a supernova, gravity
takes over so cloud condenses, getting
hotter and smaller.
Cloud becomes a T Tauri star, lowish
mass, red, buried in ISM.
Original cloud was almost certainly
rotating. As it gets smaller the rotation
rate increases (ice skater) - problem...
PHYS1142
42
Stellar Spurts
…too much angular momentum - eventually
the star would tear itself apart.
Why doesn’t it? Clues are in the spectrum.
Red line characteristic of hydrogen (H alpha).
Normally see an absorption line due to
hydrogen in the opaque stellar atmosphere. T
Tauri stars have both absorption and
emission.
Emission line comes from a wind emerging
from the star.
Sometimes spectral line is blue shifted
(Doppler effect) - indicates material coming
towards us.
Must be a large mass loss, but these stars are
supposed to be forming and accreting mass
from the surrounding dust cloud.
PHYS1142
43
Stellar Spurts
Sometimes see red shifted absorption lines due to material falling inwards to make a
growing star.
Also get ultraviolet radiation due to matter
raining down with great energy onto the
new star’s surface.
So stars are growing due to accretion and
losing mass at the same time.
What is going on?
HH Objects: Herbig and Haro discovered
bright fuzzy blobs of matter without an
apparent energy source. Complex objects
associated with gas clouds. Similar spectra
to supernova remnants.
Measurements on HH1 and HH2 showed
they were coming from a common source.
In between was a T Tauri star, buried in the
ISM, visible only in the infrared.
PHYS1142
44
Stellar Spurts and Accretion Discs
Leads to a model of an accretion disc
surrounding a star with bipolar jets.
Figs. Z15.25 & K13-16
The jets, channeled by magnetic fields,
get rid of the excess angular momentum.
We end up with a T Tauri star plus an
accretion disc plus bipolar lobes. The star
is not yet on the Main Sequence of the
HR diagram as it has still to begin fusing
hydrogen to helium.
The bipolar lobes are relatively shortlived. Is there evidence for stars with
discs? The Hubble Space Telescope has
found many such. Famous examples are
in the Orion Nebula.
PHYS1142
45