This composite
picture shows the radio
sky above an old optical
photograph
of the NRAO site in Green Bank,
WV. The former 300 Foot Telescope (the large dish standing between
the three 85 foot interferometer telescopes on the left and the 140
Foot
Telescope on the right) made this 4.85 GHz radio image, which is about
45 degrees across. Increasing radio brightness
is indicated by lighter shades to indicate how the sky would appear to
someone with a "radio eye" 300 feet in diameter. Image
credit
Thermal emission
from the Moon at
$\lambda = 850~\mu$m.
Image
credit
This
VLA image of Jupiter doesn't look
like a planetary disk at all. Most of the radio emission is
synchrotron radiation from electrons in Jupiter's magnetic field.
Image
credit
The cosmic static discovered by Karl
Jansky is dominated by diffuse
emission orginating in and near the disk of our Galaxy. The
distribution of 408 MHz continuum emission shown below in Galactic
coordinates is expected
since we are located in the disk of a galaxy similar to the edge-on
galaxy NGC 4565 shown below.
This all-sky 408 MHz continuum image (Haslam et al. 1982, A&AS, 47, 1) is shown in Galactic coordinates, with the galactic center in the middle and the galactic disk extending horizontally from it.
The
edge-on galaxy NGC 4565. We
are located in the disk of a galaxy
like this one. Image credit
Interstellar gas in our Galaxy emits spectral lines as well as continuum noise. Neutral hydrogen (HI) gas is ubiquitous in the disk. The brightness of the $\lambda \approx 21$ cm hyperfine line at $\nu \approx 1420.4$ MHz is proportional to the column density of HI along the line of sight and is nearly independent of the gas temperature. It is not affected by dust absorption, so we can see the HI throughout our Galaxy and nearby external galaxies.
Red
indicates directions of high HI column density, while blue and black
show
areas with little hydrogen. The figure is centered
on the Galactic center and Galactic longitude increases to the left.
Some of the hydrogen loops
outline old supernova remnants.
Image
credit
The
21 cm HI line traces cold hydrogen tidally torn from the galaxies in
the M81 group.
Image
credit
This false-color image of CO (J = 2-1) emission from the face-on spiral galaxy M51 was made with the Smithsonian Submillimeter Array (SMA). It reveals regions containing dense molecular gas, dust, and star formation that are optically obscured. Image credit
Some of the diffuse continuum emission from our Galaxy can be resolved
into discrete sources.
Cassiopeia A (Cas A) is the remnant of a supernova explosion that
occured over
300 years ago in our Galaxy, at a distance of about 11,000 light years
from us. Its name is derived from the constellation in which it is
seen: Cassiopeia, the Queen. A radio supernova is the explosion that
occurs
at the end of a massive star's life, and Cas A is the expanding
shell of material that remains from such an explosion. This composite
image is based on VLA data at three different
frequencies: 1.4, 5.0, and 8.4 GHz. The
material that was ejected from the supernova explosion can be seen in
this image as bright filaments. Image
credit
This
multiwavelength composite
image of the Crab Nebula shows its X-ray
(blue), optical (green), and radio (red) emission. The pulsar is
the bright point source at the center.
Image
credit
Supernova remnants and the relativistic electrons accelerated in
them account for about 90% of the $\nu \approx 1$ GHz continuum
emission from our Galaxy. Most of the remaining continuum
emission at 1 GHz is thermal emission from HII regions, hydrogen clouds
ionized by UV radiation from extremely massive stars.
The nearest large HII region is the
Orion Nebula.
The Orion Nebula is a picture book of star formation, from the massive, young stars that are shaping the nebula to the pillars of dense gas that may be the homes of budding stars. The bright central region is the home of the four heftiest stars in the nebula. The stars are called the Trapezium because they are arranged in a trapezoid pattern. Ultraviolet light unleashed by these stars is carving a cavity in the nebula and disrupting the growth of hundreds of smaller stars. Located near the Trapezium stars are stars still young enough to have disks of material encircling them. These disks are called protoplanetary disks or "proplyds" and are too small to see clearly in this image. The disks are the building blocks of solar systems.
The bright glow at upper left is from M43, a small region being shaped by a massive, young star's ultraviolet light. Astronomers call the region a miniature Orion Nebula because only one star is sculpting the landscape. The Orion Nebula has four such stars. Next to M43 are dense, dark pillars of dust and gas that point toward the Trapezium. These pillars are resisting erosion from the Trapezium's intense ultraviolet light. The glowing region on the right reveals arcs and bubbles formed when stellar winds—streams of charged particles ejected from the Trapezium stars—collide with material.
The
faint red stars near the
bottom are the myriad brown dwarfs that
Hubble spied for the first time in the nebula in visible light.
Sometimes called "failed stars," brown dwarfs are cool objects that are
too small to be ordinary stars because they cannot sustain nuclear
fusion in their cores the way our Sun does. The dark red column, below,
left, shows an illuminated edge of the cavity wall.
Image
credit
Orion's radio continuum is free-free
thermal emission from the hot
ionized hydrogen. The dusty nebula is transparent at high
radio frequencies, so all of the ionized hydrogen contributes to the
image below.
Thermal emission
from the Orion nebula. Image
credit
Thus massive, short-lived stars are
responsible for nearly all of
the radio continuum from our Galaxy.
The radio luminosities of most spiral
galaxies are proportional to
their recent star-formation rates. The nearby "starburst"
galaxy M82 has a star-formation rate about ten times that of our Galaxy
and is a correspondingly brighter radio source. Most galaxies
with little or no recent star formation (e.g., elliptical galaxies) are
radio quiet.
This
mosaic image is the sharpest
wide-angle
view ever obtained of M82. The galaxy is remarkable for its bright blue
disk, webs of shredded clouds, and fiery-looking plumes of glowing
hydrogen blasting out of its central regions. Throughout the
galaxy's center, young stars are being born 10 times
faster than they are inside our entire Milky Way Galaxy. The resulting
huge concentration of young stars carved into the gas and dust at the
galaxy's center. The fierce galactic superwind generated from these
stars compresses enough gas to make millions of more stars. In
M82, young stars are crammed into tiny but massive star clusters.
These, in turn, congregate by the dozens to make the bright patches,
or "starburst clumps," in the central parts of M82. The clusters in
the clumps can only be distinguished in the sharp Hubble images. Most
of the pale, white objects sprinkled around the body of M82 that look
like fuzzy stars are actually individual star clusters about 20
light-years across and contain up to a million stars. The rapid
rate of star formation in this galaxy eventually will be
self-limiting. When star formation becomes too vigorous, it will
consume or
destroy the material needed to make more stars. The starburst then will
subside, probably in a few tens of millions of years.
Image
credit
The strongest extragalactic radio
source in the sky is the radio
galaxy Cygnus A. The 1954 identification of this source with an
extremely distant (redshift $z \approx 0.057$, corresponding to a
distance $d \sim 240$ Mpc and a lookback time of about 700 million
years) galaxy stunned radio astronomers, who
immediately recognized that such a luminous radio source (total radio
luminosity $\approx
10^{45}$ erg s$^{-1} = 10^{38}$ W) could be detected almost anywhere in
the universe.
The angular extent of this source, about 100 arcsec, implies a linear
extent of about 100 kpc, which is much larger than the host galaxy of
stars.
The energy source is clearly not stars. Gravitational energy
released by matter accreting onto a supermassive ($M \sim
10^9\,M_\odot$) black hole in the center of the host galaxy powers this
and other luminous
extragalactic radio sources.
A
high-resolution VLA image of the
radio source Cygnus A. The bright central component is thought to
coincide with a supermassive black hole that accelerates the
relativistic electrons along two jets terminating in lobes well
outside the host galaxy.
Image
credit
This
HST gray-scale image of the
quasar 3C 273 includes radio
contours superimposed on the optical jet emission.
Image
credit
Artist's
conception
of
a
gamma-ray
burst.
Radio
observations
made with the Very Large Array, as well as the
Australia Telescope Compact Array and the Ryle Telescope, have been
combined with optical and X-ray data to show that this cosmic explosion
had a nested jet structure as shown here. The thin core of the
jet produced weak gamma-rays while the thicker envelope produced
copious radio waves. This information reveals that different types of
cosmic explosions (gamma-ray bursts, X-ray flashes, and some types of
supernovae) have the same amount of total energy and therefore share a
common origin. Image
credit
The
WMAP spacecraft near L2 beyond
the Moon.
Image
credit
Small
fluctuations
in
the
brightness
of
the
CMBR, greatly accentuated in this false-color
image.
Image
credit
The
angular power spectrum of CMBR
brightness fluctuations.
Image
credit
The
first massive stars and quasars re-ionized the universe during the
first few hundred million years. Observations of the changing HI
signal at redshifts may tell us about the end of the "dark ages" and
the epoch
of reionization.