Friday, December 29, 2006

Supernova ... Ka Boom!!!

A supernova (pl. supernovae) is a stellar explosion which produces an extremely bright object made of plasma that declines to invisibility over weeks or months. A supernova briefly outshines its entire host galaxy. It would take 10 billion years for the Sun to produce the energy output of an ordinary Type II supernova. Stars beneath the Chandrasekhar limit[1] , such as the Sun, are too light to ever become supernovae and will evolve into white dwarfs[2]

Types of Supernovae

Supernovae are divided into two basic physical types:

Type Ia. These result in some Binary-Star[3] systems in which a carbon-oxygen white dwark is accreting matter from a companion. (What kind of companion star is best suited to produce Type Ia supernovae is hotly debated.) In a popular scenario, so much mass piles up on the white dwarf that its core reaches a critical density of 2 x 109 g/cm3. This is enough to result in an uncontrolled fusion of carbon and oxygen, thus detonating the star.

Type II. These supernovae occur at the end of a massive star's lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy. If the star's iron core is massive enough then it will collapse and become a supernova.

However, these types of supernovae were originally classified based on the existence of hyderogen, Spectral Lines[4]. Type Ia do not show hydrogen lines, while Type II do.

In general this observational classification agrees with the physical classification outlined above, because massive stars have atmospheres (made of mostly hydrogen) while white dwarf stars are bare. However, if the original star was so massive that its strong Stellar Wind[5] had already blown off the hydrogen from its atmosphere by the time of the explosion, then it too will not show hydrogen Spectral Lines. These supernovae are often called Type Ib supernovae, despite really being part of the Type II class of supernovae. Looking at this discrepancy between our modern classification (based on a true difference in how supernovae explode), and the historical classification (based on early observations) shows how classifications in science can change over time as we better understand the natural world.

What Causes a Star to Blow Up?

Gravity gives the supernova its energy. For Type II supernovae, mass flows into the core by the continued making of iron from Nuclear-Fusion[6]. Once the core has gained so much mass that it cannot withstand its own weight, the core implodes (A violent inward collapse-An inward explosion). This implosion can usually be brought to a halt by neutrons, the only things in nature that can stop such a gravitational collapse (when a body falls under its own mass). Even neutrons sometimes fail depending on the mass of the star's core. When the collapse is abruptly stopped by the neutrons, matter bounces off the hard iron core, thus turning the implosion into an explosion: ka-BOOM!!!

For Type Ia supernova, the energy comes from the run-away fusion of carbon and oxygen in the core of the white dwarf.

Where Does the Core Go?

When the core is lighter than about 5 solar masses (A unit of mass equivalent to the mass of the Sun. 1 solar mass = 1 Msun = 2 x 1033 grams), it is believed that the neutrons are successful in halting the collapse of the star creating a neutron star. Neutron stars can sometimes be observed as Pulsars[7] or X-Ray binaries.

When the core is heavier (Mcore > ~ 5 solar masses), nothing in the known universe is able to stop the core collapse, so the core completely falls into itself, creating a black hole(An object whose gravity is so strong that even light can not escape from it)

To understand the phenomenon of core collapse better, consider an analogy to a rocket escaping the Earth's gravity. According to Newton's law of Gravity, the energy it takes to completely separate two things is given by:

E = G M m / r

where G is the Gravitational constant, M is the mass of the Earth, m is the mass of the rocket and r is the distance between them (the radius of the Earth). When the rocket is shot off at a given velocity v, its energy is:

E = 1/2 m v2

For the rocket to escape the Earth's gravitational field, this energy must be as least as great as the gravitational energy described in the first equation. Thus, to determine if the rocket will completely break free from the Earth's grasp, we set the two equations equal to one another and solve for v:

v = ( 2 G M / r )1/2

This result is called the escape velocity. For the Earth, the escape velocity is 11 km/sec.

Next imagine a star's central core in the role of the Earth in the above analogy. Consider what would happen if during the core collapse, the central core became so dense (i.e., the radius became very small while its mass stays the same) that something would have to travel faster than light to escape. Whenever this phenomenon occurs (i.e., Mcore > ~ 5 solar masses), the supernova creates a black hole from the core of the original star. Now the escape velocity greater than the speed of light -- 300,000 km/sec.

Where Does Most of the Star Go?

The core is only the very small center of an extremely large star that for many millions of years had been making many (but not all) of the elements that we find here on Earth. When a star's core collapses, an enormous blast wave is created with the energy of about 1028 mega-tons (The same amount energy as 1 million tons of TNT - 1 mega-ton = 4 x 1022 ergs = 4 x 1015 joules).This blast wave plows the star's atmosphere into interstellar space, propelling the elements created in the explosion outward as the star becomes a supernova remnant.

Are We Made of Stardust?

Many of the more common elements were made through nuclear-fusion in the cores of stars, but many were not as well. Because nuclear fusion reactions that make elements heavier than iron require more energy than they give off, such reactions do not occur under stable conditions that occur in stars. Supernovae, on the other hand, are not stable, so they can make these heavy elements beyond iron.

In addition to making elements, supernovae scatter the elements (made by both the star and supernova) out in to the Interstellar-Medium[8]. These are the elements that make up stars, planets and everything on Earth -- including ourselves.

How Often Do Supernovae Occur?

Although many supernovae have been seen in nearby galaxies, supernova explosions are relatively rare events in our own Galaxy, happening once a century or so on average. The last nearby supernova explosion occurred in 1680, It was thought to be just a normal star at the time, but it caused a discrepancy in the observer's star catalogue which historians finally resolved 300 years later, after the supernova remnant (Cassiopeia A) was discovered and its age estimated. Before 1680, the two most recent supernova explosions were observed by the great astronormers Tycho and Kepler in 1572 and 1604 respectively.

In 1987 there was a supernova explosion in the Large Magellanic Cloud, a companion galaxy to the Milky Way. Supernova 1987A, which is shown at the top of the page, is close enough to continuously observe as it changes over time thus greatly expanding astronomers' understanding of this fascinating phenomenon.

(this observation is updated as per 2004 - there may be some more supernovas explosion spotted between 2004-2006)

A picture of Supernova (Supernova 1987A) After & Before explosion is shown below.
The above two photographs are of the same part of the sky. The photo on the left was taken in 1987 during the supernova explosion of SN 1987A, while the right hand photo was taken beforehand. Supernovae are one of the most energetic explosions in nature, making them like a 1028 mega-ton bomb (i.e., a few octillion nuclear warheads).

Definitions:

[1]Chandrasekhar limit: A limit which mandates that no white dwarf (a collapsed, degenerate star) can be more massive than about 1.4 solar masses. Any degenerate object more massive must inevitably collapse into a neutron star

[2]White dwarf: A star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size. Typically, a white dwarf has a radius equal to about 0.01 times that of the Sun, but it has a mass roughly equal to the Sun's. This gives a white dwarf a desnsity about 1 million times that of water!]

[3]Binary star: Binary stars are two stars that orbit around a common center of mass. An X-ray binary is a special case where one of the stars is a collapsed object such as a white dwarf, neutron star, or black hole, and the separation between the stars is small enough so that matter is transferred from the normal star to the compact star star, producing X-rays in the process)

[4]Spectral Line: Light given off at a specific frequency by an atom or molecule. Every different type of atom or molecule gives off light at its own unique set of frequencies; thus, astronomers can look for gas containing a particular atom or molecule by tuning the telescope to one of the gas's characteristic frequencies. For example, carbon monoxide (CO) has a spectral line at 115 Gigahertz (or a wavelength of 2.7 mm))

[5]Stellar wind: The ejection of gas off the surface of a star. Many different types of stars, including our Sun, have stellar winds; however, a star's wind is strongest near the end of its life when it has consumed most of its fuel)

[6]Nuclear Fusion: A nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. The difference in mass is converted to energy by Einstein's famous equivalence "Energy = Mass times the Speed of Light squared". This is the source of the Sun's energy.

[7]Pulsar: A rotating neutron star which generates regular pulses of radiation. Pulsars were discovered by observations at radio wavelengths but have since been observed at optical, X-ray, and gamma-ray energies.

[8]Interstellar medium: The gas and dust between stars, which fills the plane of the Galaxy much like air fills the world we live in. For centuries, scientists believed that the space between the stars was empty. It wasn't until the eighteenth century, when William Herschel observed nebulous patches of sky through his telescope, that serious consideration was given to the notion that interstellar space was something to study. It was only in the last century that observations of interstellar material suggested that it was not even uniformly distributed through space, but that it had a unique structure.

Resources:
http://imagine.gsfc.nasa.gov/docs/science/know_l2/supernovae.html
http://en.wikipedia.org/wiki/Supernova

Thursday, December 28, 2006

Some Gyaan on Meteor

What are meteor showers?

Meteor showers are annual events, during which more shooting stars than usual can be seen over a period of several nights, each meteor appearing to point back to (or "radiate from") a particular point in the sky. These meteor showers actually occur because the earth, in its annual orbit around the sun, passes through a particular band of dust particles called a "meteoroid stream". During the course of a full solar year - when the Earth goes one full revolution around the Sun - we encounter many such meteoroid streams large and small!

Meteoroid streams are in fact the debris trails left behind by periodic comets, or in rare cases (e.g., the Geminids) by asteroids. Meteoroid streams can be visualized (in the words of Stuart Atkinson) as "rivers of crumbling comet dust". However, though streams may derive from a comet, there are forces which constantly act on the particles in meteoroid streams to move them around: thus, the meteor shower's "orbit" need not correspond with a parent comet's orbit! And it is in fact this very motion that makes meteoroid streams and their associated showers so interesting...

Where can I see meteor showers?

Meteoroid streams are always much wider than the Earth. Because of this, you will see a shower's meteors scattered over your whole sky, not for just one night but for from 3 up to 60 nights each year! Thus you don't need to face any one direction to see a meteor shower well! Nor will a meteor shower only be visible from one area of the earth. Unlike geographically narrow astronomical events like solar eclipses, lunar occultations, or bright fireballs, a meteor shower will often be visible over much of the Earth's surface!

However, not all of Earth will be able to see a given meteor shower. That is because the bulk of our globe shields some areas of Earth's surface from the impact of meteoroid particles - in effect, some area of the world map is always in the Earth's "shadow" with respect to any meteoroid stream. This "shadow" is bounded by the area of Earth in which a certain point on the Celestial Sphere (the "bowl" of the Heavens) is not visible. This special point is unique to each meteor shower, and is characterized as being the point in the sky to which all visible tracks in the heavens of meteors from the shower - no matter where they are seen in the sky, or from what point on Earth - all seem to trace back to. This point is the "radiant" of a shower!

Finally, because there is of within meteoroid streams, which the earth will "sample" as it passes through them from hour to hour, not all areas of the Earth will necessarily see the exact same show from a meteor shower! For instance, the peak activity for a particular shower may occur while it is daylight in your area of the earth! Or it may be dark during the shower's peak "maximum", but the shower radiant point may be low on your horizon, reducing the number of meteors you see - or again below the horizon, making it impossible to see any meteors from the shower at that particular hour.

Calender for 2007 Shower - http://www.imo.net/calendar/2007
================================================================
Shower Activity Max Dateλα δ vr ZHR
Antihelion Source (ANT) Jan 01 - Dec 31



30 3.0 3
Quadrantids (QUA) Jan 01 - Jan 05Jan 04 283°16 230° +49° 41 2.1 120
α-Centaurids (ACE) Jan 28 - Feb 21Feb 08 319°2 211° -59°56 2.0 5
δ-Leonids (DLE) Feb 15 - Mar 10Feb 25 336° 168° +16° 23 3.0 2
γ-Normids (GNO) Feb 25 - Mar 22Mar 14 353° 239° -50°56 2.4 4
Lyrids (LYR) Apr 16 - Apr 25Apr 22 32°32 271° +34° 49 2.1 18
π-Puppids (PPU) Apr 15 - Apr 28Apr 24 33°5 110° -45°18 2.0 var
η-Aquarids (ETA) Apr 19 - May 28May 06 45°5 338° -01°66 2.4 60
η-Lyrids (ELY) May 03 - May 12May 09 48°4 287° +44 44 3.0 3
June Bootids (JBO) Jun 22 - Jul 02Jun 27 95°7 224° +48° 18 2.2 var
Piscis Austrinids (PAU) Jul 15 - Aug 10Jul 28 125° 341° -30°35 3.2 5
South.δ-Aquarids (SDA) Jul 12 - Aug 19Jul 28 125° 339° -16°41 3.2 20
α-Capricornids (CAP) Jul 03 - Aug 15Jul 30 127° 307° -10°23 2.5 4
Perseids (PER) Jul 17 - Aug 24Aug 13 140°0 46° +58° 59 2.6 100
κ-Cygnids (KCG) Aug 03 - Aug 25Aug 18 145° 286° +59° 25 3.0 3
α-Aurigids (AUR) Aug 25 - Sep 08Sep 01 158°6 84° +42° 66 2.6 7
September Perseids (SPE) Sep 05 - Sep 17Sep 09 166°7 60° +47° 64 2.9 5
δ-Aurigids (DAU) Sep 18 - Oct 10Oct 04 191° 88° +49° 64 2.9 2
Draconids (GIA) Oct 06 - Oct 10Oct 09 195°4 262° +54° 20 2.6 var
ε-Geminids (EGE) Oct 14 - Oct 27Oct 18 205° 102° +27° 70 3.0 2
Orionids (ORI) Oct 02 - Nov 07Oct 21 208° 95° +16° 66 2.5 23
Leo Minorids (LMI) Oct 19 - Oct 27Oct 24 211° 162° +37° 62 3.0 2
Southern Taurids (STA) Oct 01 - Nov 25Nov 05 223° 52° +15° 27 2.3 5
Northern Taurids (NTA) Oct 01 - Nov 25Nov 12 230° 58° +22° 29 2.3 5
Leonids (LEO) Nov 10 - Nov 23Nov 18 235°27 153° +22° 71 2.5 15+
α-Monocerotids (AMO) Nov 15 - Nov 25Nov 22 239°32 117° +01° 65 2.4 var
Dec Phoenicids (PHO) Nov 28 - Dec 09Dec 06 254°25 18° -53°18 2.8 var
Puppid/Velids (PUP) Dec 01 - Dec 15(Dec 07)(255°) 123° -45°40 2.9 10
Monocerotids (MON) Nov 27 - Dec 17Dec 09 257° 100° +08° 42 3.0 2
σ-Hydrids (HYD) Dec 03 - Dec 15Dec 12 260° 127° +02° 58 3.0 3
Geminids (GEM) Dec 07 - Dec 17Dec 14 262°2 112° +33° 35 2.6 120
Coma Berenicids (COM) Dec 12 - Jan 23Dec 20 268° 177° +25° 65 3.0 5
Ursids (URS) Dec 17 - Dec 26Dec 23 270°7 217° +76° 33 3.0 10
================================================================
Some useful glossary

Absolute magnitude
The stellar magnitude any meteor would have if placed in the observer's zenith at a height of 100 km.
Aphelion distance
Abbreviation Q, distance of greatest heliocentric separation for a body in an eccentric orbit; Q=a(1+e).
Apollo asteroids
Asteroids having semimajor axes a>1.0 au, and perihelion distances q<1.017 class="glossary-related">See also: Asteroid, Aten asteroids, Perihelion distance, Semimajor axis
Asteroid
One of a number of objects ranging in size from sub-km to about 1000 km, most of which lie between the orbits of Mars and Jupiter; also called 'minor planet'. The preliminary designations consist of the year of discovery, an upper case letter to indicate the halfmonth in that year (A=Jan 1-15, B=Jan 16-31, ..., Y=Dec 16-31, the letter I being omitted), and a second upper case letter in sequence. When this sequence of 25 letters (with I again being omitted) has been completed it is repeated and followed by a sequential number. Permanent designations consist of numbers and names, beginning with (1) Ceres, given to asteroids for which orbits are accurately determined. Names are generally proposed by the discoverer. See also: Apollo asteroids, Aten asteroids, Meteoroid Stream
Aten asteroids
Asteroids having semimajor axes a<1.0>0.983 au. See also: Apollo asteroids, Asteroid, Semimajor axis
Comet
A diffuse body of solid particles and gas, which orbits the Sun. The orbit is usually highly elliptical or even parabolic. Comets are unstable bodies with masses of the order of 10^18 g whose average lifetime is about 100 perihelion passages. Periodic comets comprise only ~4% of all known comets. Periodic comets are designated by a number, followed by 'P/' and its name. E.g. Halley's comet has the designation 1P/Halley, the parent body of the Perseids, 109P/Swift-Tuttle. See also: Meteoroid Stream
Fireball
A bright meteor with an apparent visual magnitude of -4 mag. or brighter. See also: Meteor, Meteorite, Meteoroid, Persistent train
Geocentric
Earth-centered.
Heliocentric
Sun-centered.
Inclination
Abbreviation i., in the Solar System, the angle between an orbit and the plane of the Earth's orbit (ecliptic).
Limiting magnitude
Generally denotes the faintest star visible during an observation and evaluates the quality of the sky as well as the observing technique. The magnitude of the faintest meteor visible can be different from the stellar limiting magnitude, particularly for photographic and video observations. Visual observations assume about the same limiting magnitudes for stars and meteors. See also: Photographic observations, Video observations, Visual observations
Meteor
In particular, the light phenomenon which results from the entry into the Earth's atmosphere of a solid particle from space. See also: Fireball, Meteor Shower, Meteorite, Meteoroid
Meteor Shower
A number of meteors with approximately parallel trajectories. The meteors belonging to one shower appear to emanate from their radiant. See also: Meteor, Solar longitude, Trajectory
Meteorite
A natural object of extraterrestrial origin (meteoroid) that survives passage through the atmosphere and hits the ground. See also: Fireball, Meteor, Meteoroid, Micrometeorite
Meteoroid
A solid object moving in interplanetary space, of a size considerably smaller than a asteroid and considerably larger than an atom or molecule. See also: Fireball, Meteor, Meteorite, Meteoroid Stream
Meteoroid Stream
Stream of solid particles released from a parent body such as a comet or asteroid, moving on similar orbits. Various ejection directions and velocities for individual meteoroids cause the width of a stream and the gradual distribution of meteoroids over the entire average orbit. See also: Asteroid, Comet, Meteoroid
Micrometeorite
A small extraterrestrial particle that has survived entry into the Earth's atmosphere. The actual size is not rigorously constrained but is operationally defined by the collection procedure. Micrometeorites found on the Earth's surface are smaller than 1mm, those collected in the Stratosphere are rarely as large as 50 micro-m. See also: Meteorite
Path
The projection of the line of motion of the meteor on the celestial sphere, as seen by the observer.
Perihelion distance
abbreviation q, distance of the least heliocentric separation for a body in an eccentric orbit; q=a(1-e). See also: Apollo asteroids
Persistent train
Remaining glow due to ionization in the upper atmosphere after the passage of a meteoroid. The intensity and duration depend on the meteoroid's atmospheric entry velocity, its size, and its composition. Bright fireballs occasionally caused trains visible for several minutes. See also: Fireball
Photographic observations
The meteors are captured on a photographic film or plate. The accuracy of the derived meteor coordinates is very high. Normal-lens photography is restricted to meteors brighter than about +1mag. Multiple-station photography allows the determination of precise meteoroid orbits. See also: Limiting magnitude, Video observations
Poynting-Robertson effect
A dissipative force due to the anisotropic loss of momentum by a particle through re-radiation of solar energy. This causes aphelion collaps such that a circular orbit is soon attained; thereafter the particle spirals slowly towards the Sun. Small particles (below 1cm) are most severely affected because the force varies as the reciprocal of its size.
Radiant
The point where the backward projection of the meteor trajectory intersects the celestial sphere. More generally, the point in the sky where meteors from a specific shower seem to come from. See also: Trajectory
Radio observations
Two main methods are used, forward scatter observations and radar observations. The first are easy to carry out, but deliver only data on the general meteor activity; showers cannot be associated. The last is carried out by professional astronomers. Meteor radiants and meteoroid orbits can be determined.
Semimajor axis
Abbreviation a, half the length of the major axis of an ellipse, a standard element used to describe an elliptical orbit. See also: Apollo asteroids, Aten asteroids
Solar longitude
Angular distance along the Earth's orbit measured from the intersection of the ecliptic and the celestial equator where the Sun moves from south to north. It gives the position of the Earth on its orbit and, hence, is a more appropriate information on a meteor shower's maximum than the date. See also: Meteor Shower
Telescopic observations
Monitoring meteor activity by a telescope, preferably binoculars. This technique is used to determine radiant positions of major and minor showers, to study meteors much fainter than those seen in visual observations. See also: Visual observations
Trajectory
The line of motion of the meteor relative to the Earth, considered in three dimensions. See also: Meteor Shower, Radiant
Universal Time
The local mean time of the prime meridian. It is the same as Greenwich mean time, counted from 0 hour beginning at Greenwich mean midnight.
Video observations
This technique uses a video camera coupled with an image intensifier to record meteors. The positional accuracy is almost as high as that of photographic observations and the faintest meteor magnitudes are comparable to visual or telescopic observations depending on the used lens. Meteor shower activity as well as radiant positions can be determined. Multiple-station video observations allow the determination of meteoroid orbits. See also: Limiting magnitude, Photographic observations, Visual observations
Visual observations
Monitoring meteor activity by the naked eye. Least accurate method but easy to carry out. Large numbers of observations permit statistically significant results. Visual observations are used to monitor major meteor showers, sporadic activity and minor showers down to a ZHR of 2. See also: Limiting magnitude, Telescopic observations, Video observations, ZHR
ZHR
The number of shower meteors per hour one observer would see if his limiting magnitude is 6.5mag and the radiant is in his zenith. See also: Visual observations
Resources:
http://imo.net for Glossary & Meteor Shower Calender
http://www.meteorobs.org/showers.html for Basics of Meteor

Wednesday, December 27, 2006

Universe @ 380,000 Years Post Big Bang

Pic1: Remodelling of Big Bang showcasing big bang to birth of first star to galaxies to universe expanding at exponential rate due to dark matter/dark energy

This post is based on NASA's release on Feb 11, 2003.

The cosmic portrait–taken by NASA’s Wilkinson Microwave Anistropy Probe–shows the universe as it looked 380,000 years after the big bang, some 200 million years before any stars or galaxies had formed.

In Layman's words: "If you stand on top of Empire State Building and observe the ground, what you see by 7th floor is a time-frame by which the stars-galaxies were born, going down (going back to the past) what you see just 2" above ground is the image of baby universe (around 380,000 years old from big bang)

The probe imaged the entire sky for a full year, producing a detailed picture of the cosmic microwave background, the afterglow of the big bang. The incomparable heat of the big bang has cooled to a mere 2.73 degrees above absolute zero, but WMAP can detect temperature variations of millionths of a degree. Within these variations were the seeds that later grew into stars, galaxies and clusters of galaxies that populate the universe today.

In other findings, WMAP pegged the age of the universe at 13.7 billion years, with an error margin of only 1 percent. It also measured the contents of the universe to consist of 4 percent ordinary matter (which make up people, planets and stars), 23 percent of an unknown and unseen material called dark matter, and 73 percent of a mysterious dark energy that acts in opposition to gravity.

Pic2: A cosmic history, illustrated above, shows the first light of the universe–the afterglow of the big bang, which the WMAP observed. This light emerged 380,000 years after the big bang. Patterns imprinted on the light, which are shown in the first ellipse, encode the events that happened only a tiny fraction of a second after the big bang. In turn, the patterns are the seeds of development for galaxies we now see billions of years after the big bang.

The light we see today, as the cosmic microwave background, has traveled over 13 billion years to reach us. Within this light are infinitesimal patterns that mark the seeds of what later grew into clusters of galaxies and the vast structure we see all around us today.

Patterns in the big bang afterglow were frozen in place only 380,000 years after the big bang, a number nailed down by this latest observation. These patterns are tiny temperature differences within this extraordinarily evenly dispersed microwave light bathing the universe, which now averages a frigid 2.73 degrees above absolute zero temperature. WMAP resolves slight temperature fluctuations, which vary by only millionths of a degree.

Theories about the evolution of the universe make specific predictions about the extent of these temperature patterns. Like a detective, the WMAP team compared the unique "fingerprint" of patterns imprinted on this ancient light with fingerprints predicted by various cosmic theories and found a match.

Pic3: WMAP

Note: WMAP is named in honor of David Wilkinson of Princeton University, a world-renowned cosmologist and WMAP team member who died in September 2002.

Resources:

1) http://science.nasa.gov/headlines/y2003/11feb_map.htm
2) http://chronicle.uchicago.edu/030220/probe.shtml

Betelgeuse


Betelgeuse is the first star (other than Sun) to be spotted by Hubble Telescope. The image was taken in ultraviolet light with the Faint Object Camera on March 3, 1995.


This is also called Alpha Orionis, or Betelgeuse, it is a red supergiant star marking the shoulder of the winter constellation Orion the Hunter (diagram at right).

Other names for this star are Betelguex, Betelgeuze, Beteiguex, or Al Mankib. Betelguese and the related names derive from the Arabic phrase Ibt al Jauzah meaning "The Armpit of the Central One".

The Hubble image reveals a huge ultraviolet atmosphere with a mysterious hot spot on the stellar behemoth's surface. The enormous bright spot, more than ten times the diameter of Earth, is at least 2,000 Kelvin degrees hotter than the surface of the star.

The image suggests that a totally new physical phenomenon may be affecting the atmospheres of some stars. Follow-up observations will be needed to help astronomers understand whether the spot is linked to oscillations previously detected in the giant star, or whether it moves systematically across the star's surface under the grip of powerful magnetic fields.

Data
  • RA 05 55 10.3
  • Distance (Light Years) 427 ± 92
  • Dec +07 24 25
  • V 0.50
  • B-V +1.85
  • Spectral Type M1-2Ia-Iab
Betelgeuse (also known as alpha ori) is a very large star, an M supergiant. This is because it has evolved far from the state in which stars spend most of their lives, known as the main sequence. For stars on the main sequence, which includes our Sun, there is simple proportionality between size and mass, and also a simple scaling for luminosity. For evolved stars the situation is less simple. Betelgeuse is more than 1000 times larger than the Sun, and 50000 times as luminous, but only about 20 times as massive. Most of the light from Betelgeuse comes out in the infrared, however, which is very different from the Sun. One consequence of the advanced evolutionary state of Betelgeuse is that it probably was much more massive when it was on the main sequence, and has already lost a significant fraction of its mass (probably more than half) in a stellar wind.

There are many stars that are as massive as Betelgeuse is now, and probably many that are as massive as Betelgeuse was when it was on the main sequence. Of the 100,000,000,000 (100 billion=10^11) or so stars in our galaxy , it is estimated that approximately 1% have main sequence masses greater than 30 times that of the Sun, which is where Betelgeuse may have started out. A very crude estimate is that such stars spend 1% of their lives as supergiants, which would suggest 10,000,000 stars similar to Betelgeuse in our galaxy.

In spite of this fact, there are very few stars which are visible to the naked eye which are as large as Betelgeuse. This is simply a consequence of the fact that we can distinguish bright stars in only a small fraction of the galaxy. Another one is Mira, in the constellation Cetus. Mira is probably larger than Betelgeuse, so large that it is thought that the outer layers of the star are barely held together by gravity. Mira is known to pulsate and eject its outer layers, probably in large part because of its weak gravity. Possibly the most massive known star is eta carina, which may have been 150 times as massive as the Sun when it first formed, and may be 50 - 60 times as massive as the Sun currently. In the 1830s eta carina underwent a tremendous outburst during which it became a brilliant naked eye object and ejected an amount of gas with mass approximately equal to the mass of the Sun.

It is likely that the minimum main sequence mass for a star which will eventually make a black hole is 8 - 10 times the mass of our Sun. This is quite a bit less than Betelgeuse had when it was on the main sequence, and there are many such stars in our galaxy.

Resources:
1) Tim Kallman for Ask an Astrophysicist
2) astro.uiuc.edu
3) http://hubblesite.org
4) domofthesky.com
5) Publilshed image of Betelgeuse by NASA