Take an educational Journey to the Center of our Galaxy
The UCLA Galactic Center group is committed to promoting public understanding of current and exciting scientific research.
Just getting started? See our introduction to Galactic Center research, a Journey to the Center of the Milky Way,
by Breann Sitarski and Dr. Angelle Tanner.
Please click on a section below to start reading!
INTRODUCTION - OUR GALAXY
Schematic of the Milky way.
Credit: Oglethorpe University
What does our Galaxy look like?
We live in the Milky Way Galaxy, which is a collection of stars, gas, dust, and a supermassive
black hole at its very center. Our Galaxy is a spiral galaxy, which are rotating structures that
are flat (disk-like) like a DVD when looked upon edge-on. There is also a bulge in the middle that
consists of mostly old stars. When you look at a spiral galaxy face-on, you can see beautiful spiral
arms where stars are being born. Our solar system is in the Orion arm, and we are about 25,000 light
years (2.5 X 10^17 miles) from the very center of the Galaxy.
Since our solar system lies in one of the spiral arms,
we live in the flat plane of the Milky Way. We can actually see the dense plane of the Milky Way
stretch across the sky in dark places that do not have a lot of surrounding light pollution. The
diffuse light is the combined light from millions of stars. Some of the light from these stars
are obscured by large clouds of dust, which is why there are dark patches. Dust and gas are
necessary to form stars, and most stars are formed within the spiral arms. Note that we can't
really see the center of the galaxy with our eyes because there is dust in the way!
Annotated image of the Milky Way. The Galactic Center is unfortunately hidden by dark dust in visible light!
Center of the Milky Way
The very center of the Milky Way is known as the Galactic Center.
It has a unique collection of very exotic objects that have intrigued astronomers for many decades.
Some examples include a quiescent supermassive black hole, a collection of wispy magnetic filaments,
a few dense stellar superclusters which host mysterious and massive stars, and a family of gas streamers
spiraling toward a central dark mass.
One problem we saw earlier was that we can't look at the Galactic Center
in visible light (the light that our eyes can see) because there is dust in the way. In order to study
the Galactic center, we need to look at it at different wavelengths of light that our eyes are not sensitive to.
This can also help us uncover what is happening at this very exciting part of our Galaxy
LIGHT & WAVELENGTHS
The Galactic Center at Different Wavelengths
As you can see from the images to the right, while the optical light is dark,
the radio, sub-millimeter, mid-infrared, near-infrared, and X-ray light can all penetrate through the dust.
Different kinds of telescopes are necessary to take data at these different wavelengths. The quality of
the data one gets from a telescope is often measured by the resolution of the telescope. This is proportional
to the wavelength the data is taken at divided by the diameter of the telescope (Resolution ~ Wavelength/Diameter).
For instance, in order to obtain high-resolution images at radio (long) wavelengths, we must use an array of
telescopes that have a long effective baseline, but can be broken up into smaller telescopes. One example of this
is the Very Large Array (VLA) in Soccoro, New Mexico. However, high resolution infrared and optical images are
being produced with telescopes containing a single 10 meter mirror diameter, such as the twin Keck telescopes in
Mauna Kea in Hawaii. Let's take a look at some high-resolution images of the Galactic Center at various wavelengths
The Galactic Center was first detected in 1932 at radio wavelengths
by the pioneer of radio astronomy, Karl Jansky. However, at that point in time, radio astronomy was largely
done with a single large dish-like telescope, not arrays like the VLA. These large dishes could just detect
a strong radio source toward the Galactic Center and not mch else. Since then, some of the most striking
images of the Galactic Center have been produced by the VLA. The image to the left shows an example of
Galactic Center radio data. The two most prominent features are the non-thermal and thermal radio arcs
or "filaments" and a collection of ring-shaped supernova remnants (SNRs). The brightest part is towards
he Galactic Center. Note that the wavelength of these observations are 90 centimeters. The wavelength of light
coming from these observations is about 1.8 million times longer than the wavelengths of light our eyes can see!
The whispy radio filaments that are visible in the lower image were
first imaged by UCLA Astronomy Professor Mark Morris and his
coleague Farhad Yusef-Zadeh in the mid 1980's. These unique structures have only been observed toward the
Galactic Center and are very strongly magnetized. The emission coming from these filaments happens as a
result of charged particules spiraling at relativistic speeds around magnetic field lines.
The Very Large Array (VLA). Image Credit:
VLA 20 cm image of the radio arcs
(Lang, C. and Morris, M.)
Microwave and Sub-Millimeter
Microwave data shows emission from cool gas and dust.
The to the left shows recent results from Planck, the European Space Agency's (ESA) recent microwave
space-based mission. Since Earth's atmosphere absorbs microwaves so efficiently, all observations at
this wavelength must be done in outer space with satellites (like COBE and Planck). These results show
previously unknown groups of cold gas and a "haze" of microwaves. This is really important because
the wavelength that this image is taken at traces carbon monoxide (CO). CO is a cold molecule that
is easily detectable and traces molecular hydrogen (two hydrogen atoms bound together), which is
required for stars to form. The "haze" could come from synchrotron emission.
A Planck image of the Galaxy. Credit: ESA.
The mid-infrared wavelengths trace cold objects and are incredibly efficient in
cutting through the dust that obscures our view to the Galactic Center.
In this Chandra mid-image, we are able to see cold dust, dust-covered stars,
faint stars, and dense arcs of dust. To observe in the mid-infrared, however,
astronomers have to either send satellites into space or observe in the stratosphere, like the SOFIA observatory.
The GC in the Mid-Infrared. Credit: Chandra.
Near-infrared wavelengths are a very popular region of the
electromagnetic spectrum for observing the Galactic Center due to both thier ability to pass through
the dust along the plane as well as our ability to get high angular resolution images at these wavelengths.
Also, near-infrared detectors (which are the near-infrared version of the CCDs that are in cameras) are
progressively getting better.
Near-infrared images of the Galactic Center probe the stellar content
showing an abundant amount of stars. The earliest near-infrared images of the Galactic Center could
only resolve groups of stars, while recent observations (like those taken in the Galactic Center Group at UCLA)
c ould only resolve groups of stars.
In fact, high angular resolution near-infrared images by the Keck
telescope have been able to resolve a tiny cluster of stars orbiting the supermassive blackhole at the very
center of our Galaxy. We have also been able to obtain spectra of the stars.
This means that we take the
near-infrared light and separate it by wavelength. This allows us to determine what kinds of stars exist
in the Galactic Center, and we have found blue supergiants, red giant stars, dust obscured stars,
and so on. With bigger telescopes on the way
(like the Thirty Meter Telescope), there is much more near-infrared research ahead!
Keck high-resolution image of the Galactic Center. Sgr A* corresponds to the central black hole.
4 micron image of the Galactic Center.
It is nearly impossible to observe anything at the Galactic Center
at optical wavelengths. While our telescopes to have the resolution to do so, the large amount of
starlight-obscuring dust along the plane of the Galaxy causes any optical light from the Galactic
Center to be extincted by 30 orders of magnitude (1 X 10^30).
An optical image of the Galactic plane.
The image to the left shows X-ray emission from the
Galactic Center taken with the Chandra X-Ray observatory. The points of light seen in
this image show hot white dwarfs, neutron stars, and black holes, all swimming in a
hot gas many millions of degrees. The brightest patch in the center is the supermassive
black hole. Star clusters are also visible that host massive, x-ray emitting stars.
The gas here glows in X-ray light because it has been heated to a temperature of millions
of degrees by shock waves produced by supernova explosions and perhaps by colliding winds
from young massive stars. The Galactic Center is truly an energetic region.
An image of the Galactic plane in X-ray.
GAS & DUST
What is happening to the gas in the Galactic Center?
As we saw before, we can trace ionized gas at radio wavelengths.
High-resolution observations of the Galactic Center at radio wavelengths have revealed a structure called
the "mini-spiral." The mini-spiral has several different parts--the Northern Arm, the Wester Arc, and the Eastern Arm
VLA image of the Galactic Center at 6 cm.
Motion of the gas
The motion of the gas in the mini-spiral can be determined using
the Doppler shift of emission lines,
particularly Ne II and Bracket-gamma hydrogen lines. These studies show that the gas along the Northern
Arm is falling in toward, or obriting around, the central black hole. Current evidence suggests that the three
arms are orbiting the supermassive black hole according to Kepler's laws, and that the gas in the Northern Arm is
actually interacting with a nearby source. The Eastern Arm is actually colliding with the Northern arm flow, and
that the gas is photoionized--electrons are being stripped off (Zhao et al. 2009).
1.3 cm image of the mini-spiral. Source: Zhao et al. 2009.
The Circumnuclear Disk (CND)
In addition to having different arcs and arms, the mini-spiral is surrounded by a
thick ring of molecular material called the Circumnuclear Disk (CND). The origin of this disk is unclear,
but it is a ring, or a torus, with an inner radius of about 1 parsec and an outer radius of several parsecs
(Note: 1 parsec is 1.917 X 10^13 miles!). This disk's rotation follows Kepler's laws, and it is heated by both
central region of old stars and the ultraviolet (short wavelength) flux of bright young stars that are
near the black hole.
Molecular gas (HCN) in the circumnuclear disk. Credit: Leo Blitz, University of Maryland.
25" X 25" H & K image of the Central Parsec Cluster. Credit: Gemini.
What sort of stars are at the Galactic Center?
High resolution near-infrared imaging has led to the discovery of
three stellar superclusters at the Galactic Center. Since near-infrared wavelengths cut through the dense
dust between Earth and the Galactic Center, we are able to see these superclusters.
They include the Central Parsec, Quintuplet, and Arches clusters.
The Arches cluster is a densely packed star cluster,
which is surprising due to it's closeness to the supermassive black hole at the center of our Galaxy.
Since black holes have a strong gravitational field, they affect the area around them with tidal forces.
The Arches cluster's proximity should cause the cluster to be torn apart. The fact that it is still around means
that it is probably a young cluster. Members of the Galactic Center Group at UCLA discovered circumstellar
disks in the Arches Cluster (the presence of a circumstellar disk means that a star is really young,
and it is in the process of forming), which supports the hypothesis that this cluster is very young (Stolte et al. 2010).
The Quintuplet Cluster is named after the five red stars in the image to the left. These particular stars do not
have any observable emission or
absorption lines in their stellar spectra, which has kept their identity a mystery.
HST image of the Arches and Quintuplet clusters. Credit: NICMOS.
The Quintuplet cluster
In addition to the young stars found in the Arches
cluster and the peculiar stars found in the Quintuplet cluster, there is a large number of late-type
(not very massive and fairly cold, spectral type K and M) stars at the Galactic Center.
These were discovered by looking for CO absorption bands in their stellar spectra. These are therefore cold stars,
and they look red in the top Gemini image (red colors mean that the object is cool, while blue and white objects are hot).
The brightest star in the Gemini image is known as IRS 7, and it is an M supergiant. Most of the late-type stars are giants,
supergiants, or more evolved giant stars called AGB, OH-IR, and Mira viarable stars. The M and K sueprgiants were formed 3 - 7
million years ago, while the AGB stars are about 100 million years old. There is a lack of late-type stars in the central
10 arcseconds of the Galaxy, close to the supermassive black hole. Astronomers are trying to determine why this is the case,
and where the young, early-type stars that exist there came from--if they were formed in the Galactic
Center or if they were formed somewhere else and moved to the center of our Galaxy.
The young, massive stars
The young, massive, early-type (O, A, and B spectral class)
stars are the brightest stars in the Galactic Center and contribute most of the ultraviolet radiation
that ionizes the surrounding gas seen in the mini-spiral. These stars have Helium (He I) and Hydrogen (H I)
emission lines in their spectra, which are come from the fast winds flowing away from the stars. The most noticeable
cluster of these stars are in the IRS 16 cluster, which are located close to Sgr A*. T
hese stars are also 3 - 7 million years old. The Galactic Center tends to host, on average, more massive stars than other clusters found in the local neighborhood.
This is probably due to the extreme conditions found at the Galactic Center created by the supermassive black hole,
the large magnetic field, and the UV radiation from early-type stars.
K-band mosaicked image of the Galactic Center taken at Keck Observatory.
This image also shows young stars at the Galactic Center.
OUR SUPERMASSIVE BLACK HOLE
Is there a black hole at the center of the Galaxy?
As we zoom into the very core of the Galactic Center, our field of view shrinks to a mere 5 arcseconds
(one thousandth of a degree). At radio wavelengths, the brightest feature of this region is the point-like radio
source Sagittarius A* (pronounced "Sag A star"). This source is a compact object, and approximately one
Astronomical Unit (1 AU is about 93 million miles) in size, which is much smaller than our solar system
(Neptune is 2.8 billion miles from the Sun). At near-infrared wavelengths, this point source in the radio is
not clearly seen. Astronomers have seen pulsation near the
radio position of Sgr A* in the near-infrared, which they attribute to this radio source flaring.
In 1974, Sir Martin Rees proposed the idea that supermassive black holes could exist within the centers of active
galactic nuclei or quasars. In that same year,
Balick and Brown made the conenction between their radio detection of Sgr A* and other known active galactic nuclei.
In the past 20 years, astronomers have collected enough evidence through the observed motions of gas and stars to convince
ourselves that something very massive lurks at the center of our galaxy. The first dynamical evidence came from the motions
of the ionized gas streamers of the mini-spiral orbiting Sgr A*. Using the velocities of the gas estimated from the Doppler
shift of spectral lines, astronomers estimated that a mass of six million solar masses must lie within 10 arcseconds of
Sgr A*. This did not explicitly prove the existence of a black hole since that amount of matter could be accounted for by a high density of stars within such a large volume.
The center of our Galaxy, as seen in the radio.
Credit: Farhad Zadeh, VLA, NRAO, APOD
The mini-spiral in the GC. It is centered on Sgr A*.
The Groups studying the Galacic Center
Since 1995, high-resolution near-infrared studies have observed a compact cluster of early-type stars surrounding
the radio position of Sgr A*. These stars have very large proper motions (they are moving across the sky very quickly)
considering their 24 million light year distance from the Earth. The two main groups devoted to tracking these stars
include Andrea Ghez and others at UCLA, who use the 10-m Keck telescope on Mauna Kea, Hawaii, and Reinhard Genzel and
Andreas Eckart who use the 8-m VLT telescopes in Chile. Both groups take advantage of the high spatial resolution and
sensitivity of these large telescopes to track the positions of the stars within the cluster using near-infrared images collected once or twice a year.
The twin Keck 10 meter telescopes. Credit: Swinburne University of Technology
Despite the large diameters of the Keck and VLT telescopes, air turbulence in the Earth's atmosphere blurs the images
taken at the telescopes. The atmosphere has a lot of molecules that are colliding into each other and getting heated up.
On really hot days, we can see the heat waves coming up off the ground, or if you look at a flame, you can see the heat
influencing the air around it. This is what happens in our atmosphere. In order to correct for it, astronmers are now using
Adaptive Optics (AO) systems, which increases the sensitivity of observations. AO systems use a deformable mirror
that mimics the shape of the incoming lightwave and corrects for the atmospheric turbulence before the data is recorded.
In order to see the individual stars in the GC, Adaptive Optics is necssary.
Stellar Orbits about the Black Hole.
Very accurate stellar positions can be estimated in order to keep track of the motions of the stars in the compact
central cluster, which are zipping around Sgr A* at speeds up to 3 million miles per hour! Using Kepler's laws of
motion, the orbital velocities and the positions of the bright stars an be used to estimate the mass that must be
contained within their orbits. The resulting enclosed mass is 4.6 ± 0.7 X 10^6 solar masses--4.6 million times the
mass of our Sun! This large mass combined with the minute size of Sgr A* in radio emission suggests taht the stars
must be swiftly circling around a supermassive black hole.
The orbits of the stars in the Galactic Center from 1995 - 2013.
Black Holes in other Galaxies
Recent observations of nearby galaxies reveal that such supermassive black holes are not unique to the MIlky Way.
The formation of such a large black hole and how it affects the evolution of its host galaxy are not well understood.
In the case of Sgr A*, there is a mysterious absence of the high energy emission (X-rays and UV radiation) often
observed from active galactic nuclei. The image to the left is of an active galactic nucleus in Cen A.
There is a lot of activity at long wavelengths and short,
energetic wavelengths. This is completely different than our galactic nucleus, Sgr A*.
Sgr A* may have been more energetic in the past.
Hope you have enjoyed this Galactic Journey and now have some understanding of what's going on in the center of our
Thank You - The Galactic Center Group.
An active galactic nucleus (Cen A). There is a lot of activity at long wavelengths and short,
energetic wavelengths. Credit: NASA