PHY2083
ASTRONOMY
The Sun, our nearest star
The E/M spectrum
Fermi (Gamma Rays)
Chandra (X-rays)
Spitzer (Infrared)
Herschel (far-IR/sub-mm)
Xray
Infrared
UV
Radio
Visible
Combined
XRay
XRay
The Sun
Fairly average star, but
with a very unique
feature: it is extremely
nearby, only 1.49 x
1011m!
Only 8 light mins. away,
c.f. our next nearest
neighbour, alpha Cen ~
300 000x further away
(4.3 ly)
The Sun
Radius ~ 700 000 km
Angular size = 0.5 deg
Use distance and apply
basic trig. to get the
radius!
SOHO
SDO
STEREO
Hinode
RHESSI
The Solar Spectrum
The Sun
The Photosphere
“sharp surface”
layer where optical
photons originate
base of photosphere
Absorption lines
Absorption lines:
Produced by a
continuous source with
cooler gas in front. The
cooler gas preferentially
absorbs at characteristic
wavelengths, causing
dark lines.
Darkest part of line from regions
higher up in the photosphere where the
gas is cooler.
Sunspots
dark patches on
the surface of
the sun. First
observed by
Galileo
tens of
thousands of
km across
Sunspots show a dark centre “umbra” surrounded by a
lighter region “penumbra”. This gradation is due to a
change in temperature.
Sunspots are cooler than their surroundings
Umbra: ~ 4500K
Penumbra ~ 5500 K
Surroundings ~ 5800 K
Sunspots appear dark because they
are seen against a bright background.
If the background were removed, then
they would glow, in the same way as
any other hot object!
What causes a sunspot?
Why are sunspots are cooler than their surroundings?
Linked to the magnetic field of the sun
Zeeman
splitting
What causes a sunspot?
Why are sunspots are cooler than their surroundings?
Linked to the magnetic field of the sun
The B-field in a sunspot is ~1000x
greater than in the surrounding
regions, with field lines roughly
perpendicular to the surface.
They are cooler because the B-field
blocks the flow of hot gas from the
centre to the surface
What causes a sunspot?
Why are sunspots are cooler than their surroundings?
Linked to the magnetic field of the sun
They are cooler because the B-field
blocks the flow of hot gas from the
centre to the surface
T lower => Pressure lower => why don’t sunspots
sink towards the interior?
Sunspots usually occur in pairs, and have opposite polarity
Rotation of the sun
Solar minimum
Solar maximum
• Sunspots have been observed for over 2
centuries
• Number of sunspots is approximately
periodic (~11 yrs)
• Average latitude is also periodic
• Individual sunspots live for a few weeks
The Solar Cycle: butterfly diagram
Faculae & Granulation
Granulation
Flares
Occur near
active regions
and last for a few
mins.
Release vast
amounts of
energy ~1020 J/s
Prominences
Prominences
B-field lines twisted due to
differential rotation
Prominences: ~ 100 000 km
Chromosphere
Corona
Spicules
Occur typically a few
1000km above the
photosphere in the
“chromosphere”.
Jets of plasma several
thousand km long
Spicules
Corona
Outermost layer of the sun, only visible during solar
eclipses, or from space.
Spectrum of corona completely different! New
element “Coronium”
Atoms highly ionised e.g lines of FeXIV (lost 13 out of
26 electrons) c.f. photosphere, only lost 1-2.
Corona
T_corona >> T_photosphere; T_corona ~ 3 000 000K!
Corona ~ 10 000 km above photosphere
Why? How?
Coronal mass ejection (CME)
Solar Wind
~10 million km above
the photosphere, the
coronal gas can
escape the Sun’s
gravity.
Solar wind: fast moving particles ~
500 km/s => reach
the Earth (1AU) in a
few days
Solar Wind
Solar Wind
The sun is in effect “evaporating”.
The solar wind carries away ~ 2 million tonnes of matter
per second!
c.f. mass of sun 2 x 1030 kg (less than 0.1% since the
formation of the solar system ~ 4.6 billion yrs ago)
Solar Wind
Aurora
Aurora (Belfast)
Hydrostatic Equilibrium
Forces on an
atmospheric element
Pouter
Fgrav
Pinner
Equation of Hydrostatic
Equilibrium
dP
Gm(r)ρ(r)
=−
dr
r2