Keep and Eye On Hypergiant Rho
Keep an Eye on Hypergiant Rho Rho
Mercury, January/February 2004 Table of Contents
Image courtesy of A Lobel.
by Alex Lobel
An eruption in 1946 of mighty Rho Rho put the astronomy community on guard, and recent, exciting changes in the star may portend something big and explosive for it in the near future.
She goes without a proper name, but recently the 17th brightest star of the northern constellation Rho is drawing the attention of amateur and professional astronomers worldwide. In the spring of 2000 Rho Rho, or r Cas, brightened up to magnitude 4.0, then dimmed to an astonishing 5.3 over the next half year, while changing its usual yellowish-white color to the red-orange glare of Betelgeuse (a Orionis). Such a rapid, extraordinary change was also observed for the star in 1946, bringing it to the attention of astronomers everywhere.
Various types of variable stars are known to change their visual brightness in a rather predictable wayâ€â€stars such as Mira (o Ceti) and Algol (b Persei). And the R Coronae Borealis stars can suddenly dim by several magnitudes; they are much fainter and less intrinsically luminous than r Cas, however.
Indeed, shining at about half a million times the Sun’s luminosity, Queen r Cas is known to be one of the most luminous, cool stars of our Galaxy. Tucked away in the Orion spiral arm of the Galaxy, at an approximate distance of ten thousand light-years, it is possibly the most distant star with a surface temperature comparable to that of our Sun that can easily be observed with the unaided eye.
If you enjoyed this excerpt from a feature article and would like to receive our bi-monthly Mercury magazine, we invite you to join the ASP and receive 6 issues a year.
Visit Aoleon The Martian Girl
pokemon store
Dragonball Z Store
Harry Potter Store
Neopets Store
Smash bros Store
Astronomy Store
Hubble Finds Ghostly Ring of dark matter
Hubble Finds Ghostly Ring of dark matter
05.15.07
Visit Aoleon The Martian Girl
The most common substance in the universe is called dark matter. It doesn’t shine or reflect light. We can’t even see it.
Image right: This Hubble Space Telescope composite image shows a ghostly “ring” of dark matter in the galaxy cluster Cl 0024+17. The ring-like structure is evident in the blue map of the cluster’s dark matter distribution. The map is superimposed on a Hubble image of the cluster. The ring is one of the strongest pieces of evidence to date for the existence of dark matter, an unknown substance that pervades the universe. Click image to enlarge. Credit: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University)
It is an invisible substance composed of atoms that are far different from those that make up the universe’s normal matter, such as stars and galaxies.
In fact, if you drove into a wall made of dark matter, you wouldn’t crack a headlight or inflate an airbag. You wouldn’t even know it happened. But what happens to dark matter during a collision?
astronomers using NASA’s Hubble Space Telescope got a first-hand view of how dark matter behaves during a titanic collision between two galaxy clusters. The wreck created a ripple of dark matter, which is somewhat similar to a ripple formed in a pond when a rock hits the water.
The ring’s discovery is among the strongest evidence yet that dark matter exists. astronomers have long suspected the existence of the invisible substance as the source of additional gravity that holds together galaxy clusters. Such clusters would fly apart if they relied only on the gravity from their visible stars. Although astronomers don’t know what dark matter is made of, they hypothesize that it is a type of elementary particle that pervades the universe.
Visit Aoleon The Martian Girl
The ring-like structure is evident in a composite image of the cluster made from Hubble observations. The ring can be seen in the blue map of the cluster’s dark matter distribution, which is superimposed on an image of the cluster.
The Hubble astronomers say it is the first time they have detected dark matter as having a unique structure that is different from the gas and galaxies in the cluster. The researchers spotted the ring unexpectedly while they were mapping the distribution of dark matter within the galaxy cluster Cl 0024+17 (ZwCl 0024+1652), located 5 billion light-years from Earth. The ring measures 2.6 million light-years across.
Image left: This rich galaxy cluster, catalogued as Cl 0024+17, is allowing astronomers to probe the distribution of dark matter in space. The blue streaks near the center of the image are the smeared images of very distant galaxies that are not part of the cluster. The distant galaxies appear distorted because their light is being bent and magnified by the powerful gravity of Cl 0024+17, an effect called gravitational lensing. Click image to enlarge. Credit: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University)
Although astronomers cannot see dark matter, they can infer its existence in galaxy clusters by observing how its gravity bends the light of more distant background galaxies, a powerful effect called gravitational lensing. The blue streaks near the center of another Hubble image of the cluster are the distorted shapes of more distant galaxies, whose light was bent and magnified by the powerful gravity of Cl 0024+17.
The collision between the two galaxy clusters, the astronomers explained, created a ripple of dark matter that left distinct footprints in the shapes of the background galaxies. It’s like looking at the pebbles on the bottom of a pond with ripples on the surface. The pebbles’ shapes appear to change as the ripples pass over them. So, too, the background galaxies behind the ring show coherent changes in their shapes due to the presence of the dense ring. Although the invisible matter has been found before in other galaxy clusters, astronomers say it has never been detected to be so largely separated from the hot gas and the galaxies that make up galaxy clusters.
The astronomers found previous research that suggested the cluster had collided with another cluster 1 to 2 billion years ago. They then created computer simulations of galaxy cluster collisions. The simulations show that when the two clusters smash together, the dark matter falls to the center of the combined cluster and sloshes back out. As the dark matter moves outward, it begins to slow down under the pull of gravity and pile up, like cars bunched up on a freeway.
Visit Aoleon The Martian Girl
Chandra Discovers the Powerful Wind of a Microquasar
===============================================================
Chandra Discovers the X-ray Signature of a Powerful Wind from a
Galactic Microquasar
===============================================================
Digital images and movies are available on the World Wide Web at
http://www.astro.psu.edu/users/niel/cirx1/cirx1.html
NASA’s Chandra X-ray Observatory has detected, for the first time in
X-rays, a stellar fingerprint known as a P Cygni profile–the
distinctive spectral signature of a powerful wind produced by an
object in space. The discovery reveals a 4.5-million-mile-per-hour
wind coming from a highly compact pair of stars in our galaxy, report
researchers from Penn State and the Massachusetts Institute of
Technology in a paper they will present on 8 November 2000 during a
meeting of the High-Energy Astrophysics Division of the American
Astronomical Society in Honolulu, Hawaii. The paper also has been
accepted for publication in The Astrophysical Journal Letters.
“To our knowledge, these are the first P Cygni profiles reported in
X-rays,” say researchers Niel Brandt, assistant professor of
astronomy and Astrophysics at Penn State, and Norbert S. Schulz,
research scientist at the Massachusetts Institute of Technology. The
team made the discovery during their first observation of a
binary-star system with the Chandra X-ray Observatory, which was
launched into space in July 1999. The system, known as Circinus X-1,
is located about 20,000 light years from Earth in the constellation
Circinus near the Southern Cross. It contains a super-dense neutron
star in orbit around a normal fusion-burning star like our Sun.
Although Circinus X-1 was discovered in 1971, many properties of this
system remain mysterious because Circinus X-1 lies in the galactic
plane where obscuring dust and gas have blocked its effective study
in many wavelengths.
The P Cygni spectral profile, previously detected primarily at
ultraviolet and optical wavelengths but never before in X-rays, is
the textbook tool astronomers rely on for probing stellar winds. The
profile looks like the outline of a roller coaster, with one really
big hill and valley in the middle, on a data plot with velocity on
one axis and the flow rate of photons per second on the other. It is
named after the famous star P Cygni, in which such profiles have been
observed for over one hundred years. “When you see a P Cygni
profile, you immediately know the object you are observing is
producing a powerful outflow,” Brandt says. Chandra is the first
X-ray observatory capable of capturing data of sufficiently high
resolution to reveal an X-ray P Cygni profile.
Brandt and Schulz say their discovery occurred because they were able
to use Chandra continuously for one-third of a day to observe
Circinus X-1, plus its signal in X-rays is generally very bright,
partly because it is relatively nearby in our own galaxy. P Cygni
lines at ultraviolet or optical wavelengths had not been previously
seen from Circinus X-1 because a large amount of dust in the galactic
plane lies between Earth and this system and this dust is an
efficient absorber of ultraviolet and optical light. However, the
energetic X-rays created by Circinus X-1 could easily penetrate
through the obscuring dust and gas–similar to the way medical X-rays
on Earth can penetrate through people’s bodies. “We were hoping to
detect some kind of X-ray line emission from the accreting neutron
star in Circinus X-1, but it caught us totally by surprise to observe
a complex emission structure like a P Cygni profile in high-energy
X-rays.” Schulz says. “This detection clearly marks a new area in
X-ray Astrophysics, where we will be able to study dynamical
structures in the Universe like we currently do at ultraviolet or
optical wavelengths.”
Brandt and Schulz used two of Chandra’s instruments, known together as
the High-Energy Transmission Grating Spectrometer (HETGS), to detect
the X-rays and produce a high-resolution X-ray spectrum of Circinus X-1.
This spectrum is analogous to the rainbow we can see at optical
wavelengths. “Chandra’s X-ray spectrum is 50 times more detailed
than previous X-ray observatories could obtain,” Schulz says. First,
the super-fine transmission gratings acted like a prism to separate
the X-rays into discrete energy bands. Then, the Advanced CCD
Imaging Spectrometer (ACIS) was used as a camera to record the X-ray
spectral data, which computers processed and plotted onto a graph,
revealing the P Cygni signature. Specific elements, such as silicon
or iron, emit specific X-ray wavelengths, revealing their presence in
the emitting material to astronomers.
Before the observation with Chandra, astronomers knew the force of
gravity in an X-ray binary system strips material off the surface of
the normal star and then pulls this material toward the surface of
the super-dense neutron star, forming a relatively flat spiraling
cloud of gas called an accretion disk. The detailed Chandra data
revealed, in addition, that the radiation and rotational forces in
the Circinus X-1 disk are blasting some of the inward-spiraling gas
back out into space in a powerful wind, which creates the P Cygni
lines in the object’s spectrum.
P Cygni profiles carry much diagnostic information that is hard to
obtain in other ways–such as how fast the wind is moving, how much
material it contains, how dense it is, and its chemical composition.
“The wind coming out of Circinus X-1 is composed of gas that contains
highly ionized atoms of silicon, neon, iron, magnesium, and sulfur,
and its peak observed velocity is about 4.5 million miles per
hour–so fast it would cross the entire radius of the Earth in about
three seconds,” Brandt reports.
The astronomers used Doppler techniques that detect positive
velocities from material moving away from Earth, with signals shifted
toward the red end of the spectrum, and negative velocities from
material that is coming toward Earth, with signals shifted toward the
blue end of the spectrum. “We learned these two stars clearly
interact dramatically with each other while this wind is blowing
outward at high velocity, which appears to be causing certain
properties of the wind to change over time,” Schulz says.
The researchers produced a time-lapse movie of one of their spectra,
which is available on the World Wide Web, along with other
information about the discovery, at
http://www.astro.psu.edu/users/niel/cirx1/cirx1.html
Atoms irradiated with energetic X-rays can emit as well as absorb
them at specific wavelengths. Whether astronomers observe emission
or absorption depends on the state and environment of the irradiated
atoms, so these processes carry vital information about the emitting
and absorbing material. Regarding the time-lapse movie, Schulz
commented “You can see this profile flipping up and down between a
strong emission line on the red side and a strong absorption line on
the blue side. We don’t yet fully understand what this means, but it
does indicate the dynamic nature of this system. We see indications
that sometimes either the emitting or the absorbing region gets
obscured by matter so thick that not even X-rays can penetrate it.”
The researchers say one reason their discovery that Circinus X-1 has
a high-velocity wind is important is that this small two-star system
now has striking similarities with a type of luminous active galaxy
known as a broad-absorption-line quasar. Broad-absorption-line
quasars are galaxies containing a violent centers powered by
supermassive black holes. “This type of galaxy has an accretion disk
circling its black hole plus very powerful winds created when
radiation pushes material off of the disk and out into space,” Brandt
says. “The disk winds from broad-absorption-line quasars create P
Cygni lines in the spectra of these objects. Circinus X-1, with the
newly detected X-ray P Cygni profiles, appears in many ways to be a
microscopic version of a broad-absorption-line quasar.”
“Although a typical AGN has a roughly ten-million-solar-mass black
hole at its center while the Circinus X-1 system has a neutron star
only slightly more massive than our Sun, both systems must obey the
same laws of physics,” Brandt says. “Gas is gas and gravity is
gravity and that’s all there is to it–you put gas and gravity
together and they make a disk and often, apparently, a disk-generated
wind.” The researchers hope X-ray P Cygni profiles will be found to
be a fairly common property of X-ray binaries containing neutron
stars and black holes. “If we can find X-ray P Cygni profiles in
more systems, we can learn a great deal about the geometry and the
dynamics of the winds these systems emit,” Schulz says. “Due to the
penetrating nature of X-rays, X-ray P Cygni lines have the
significant advantage that they can be used to probe winds even from
systems that are heavily obscured by dust along the line of sight.”
The High-Energy Transmission Grating Spectrometer was built by the
Massachusetts Institute of Technology with Bruno Rossi Professor
Claude Canizares as Principal Investigator. The ACIS X-ray camera
was conceived and developed for NASA by Penn State and the
Massachusetts Institute of Technology under the leadership of Gordon
Garmire, Evan Pugh Professor of Astronomy and Astrophysics at Penn
State. The observation of Circinus X-1 was part of the first round
of Chandra’s guest observer program. The guest observer program is a
competitive one open to the World science community.
NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages
the Chandra program. TRW Inc., Redondo Beach, California, is the
prime contractor for the spacecraft. The Smithsonian’s Chandra X-ray
Center controls science and flight operations from Cambridge,
Massachusetts.
To follow Chandra’s progress, visit the Chandra site at:
http://Chandra.harvard.edu and http://Chandra.NASA.gov
This research was supported by the Chandra X-ray Center, the Alfred
P. Sloan Foundation, and the Smithsonian Astrophysical Observatory.
This is a joint press release from Penn State and the Massachusetts
Institute of Technology
Contacts:
Niel Brandt: 814-865-3509, niel@astro.psu.edu
Norbert S. Schulz: 617-258-5767, nss@space.mit.edu
Barbara K. Kennedy (PIO at Penn State): 814-863-4682, science@psu.edu
Elizabeth Thompson (PIO at MIT): 617-253-2700 or 617-258-5402, thompson@mit.edu
