Protostar
Example: TW Hydrae
Protostars and very young stars are usually surrounded by disks of dust and gas. Some of this matter will fall onto the young star, some may form into planets, and the remainder will be blown away by intense radiation from the star. In TW Hydrae, the X-ray spectrum provides strong evidence that this very young star is pulling in matter from a circumstellar disk. X-rays are produced as the infalling matter collides with the surface of the star.
Illustration: NASA/CXC/M.Weiss
Stellar Nursery
Example: Orion Nebula
Most stars form as members of star clusters created by the collapse of cold (10 degrees above absolute zero), dense clumps of gas and dust embedded in much larger clouds of cold gas and dust. At a distance of about 1,800 light years, the Orion Nebula cluster is the closest large star-forming region to Earth. Chandra’s image shows about a thousand X-ray emitting young stars in the Orion Nebula star cluster. The X-rays are produced in the hot, multimillion-degree upper atmospheres of these stars. (The dark diagonal lines and the streaks from the brightest stars are instrumental effects.)
Chandra X-ray Image of Orion Nebula
Credit: NASA/CXC/Penn State/E.Feigelson & K.Getman et al.
Brown Dwarf
Example: TWA 5B
An object that has a mass of less than about 8% of the mass of the Sun cannot sustain significant nuclear fusion reactions in its core. This marks the dividing line between red dwarf stars and brown dwarfs. The brown dwarf TWA 5B has a mass estimated at about 3% that of the Sun. The turbulent interiors of young brown dwarfs can combine with rapid rotation to produce a tangled magnetic field that can heat their upper atmospheres, or coronas, to a few million degrees Celsius. The X-rays from TWA 5B are likely due to this process.
Chandra X-ray Image of TWA 5B
Credit: NASA/CXC/Chuo U./Y.Tsuboi et al.
Red Dwarf
Example: Proxima Centauri
The nearest star to Earth, Proxima Centauri, is the most common type of star in the Galaxy – a red dwarf star. Red dwarfs have a mass between approximately 8% and 50% of the mass of the Sun. Because of their low mass, nuclear fusion reactions that consume all of the hydrogen in the core of red dwarfs can take 20 billion years or more – longer than the estimated 14 billion-year age of the Universe. A red dwarf has a turbulent interior that tangles the magnetic field and heats the star’s corona, sometimes explosively. For this reason, red dwarfs are observed to be strongly variable X-ray sources.
An artist's illustration depicts the interior of a low-mass star. Such stars have different interior structures than our Sun, so they are not expected to show magnetic activity cycles. However, astronomers have discovered that the nearby star Proxima Centauri defies that expectation and shows signs of a 7-year activity cycle.
Credit: NASA/CXC/M.Weiss
White Dwarf
Example: Sirius B
The central star of a planetary nebula will eventually collapse to form a white dwarf star. In the white dwarf state, all the material contained in the star, minus the amount blown off in the red giant phase, will be packed into a volume one millionth the size of the original star. An object the size of an olive made of this material would have the same mass as an automobile! For a billion or so years after a star collapses to form a white dwarf, it is white-hot with surface temperatures of about 20,000 degrees Celsius. After that it slowly cools to become an undetectable "black dwarf". The bright source in this image is the white dwarf Sirius B. (The spike-like pattern is an instrumental artifact.)
Credit: NASA/SAO/CXC
Sun-like Star
The Sun and other stars are balls of gas that shine as a result of nuclear fusion reactions that release energy deep in their interiors. The Sun is now in a long-lived phase of its evolution wherein nuclear reactions are converting hydrogen to helium in the central core. In a thick outer shell of the Sun, the gas is in a state of rolling, boiling turmoil called convection. This up and down motion, coupled with the Sun’s rotation, twists the magnetic field and increases its strength. Twisted, magnetized loops of hot gas rise high above the surface of the Sun, where they make up the corona – the outermost layers of the Sun’s atmosphere. The Sun’s X-rays (too intense for Chandra to observe) are produced in these loops, which can also be the site of solar flares.
Credit: NASA/SDO
Main Sequence
Hydrogen Burning Core
Once a young protostar has accreted all of the gas and dust that it can from the cloud from which it was born, it may be massive enough to burn hydrogen in its core and shine as a star. If and when this happens, it becomes a zero-age main sequence star. The main sequence is defined as the part of a star's lifetime spent burning hydrogen at its core; the start of its main-sequence lifetime is the point at which hydrogen burning first begins, and the end is defined by the point at which it runs out of hydrogen in its core. The amount of time spent on the main sequence can vary from star to star too; the main sequence lifetime is mainly a function of a star’s mass. The Sun is now in this long-lived phase of its evolution.
Hubble Optical Image of Orion Nebula, Close-up.
Credit: NASA/STScI/Rice Univ./C.O'Dell et al.
Red Giant
Example: Beta Ceti
When the hydrogen in the star's core is used up, the energy flow from the core of the star stops, the central regions of the star will slowly collapse and heat up. Nuclear reactions in a shell of gas outside the core will provide a new source of energy, and cause the aging star to expand outward in the "red giant" phase.
A solar-type star becomes a red giant after nuclear fusion reactions that convert hydrogen to helium have consumed all the hydrogen in the core of the star. The core collapses until hydrogen fusion begins in a hot, gaseous shell around the core. Energy generated by hydrogen fusion in the shell causes the star's diameter to expand about a hundredfold. As the gas expands, it cools, and the star becomes a red giant. During this period, the star emits X-rays weakly. Eventually the core contracts and heats until fusion reactions begin to convert helium to carbon, and the star becomes a core-helium-burning giant. Beta Ceti is an example of such a giant star, which can be X-ray active.
Credit: NASA/CXC
Blackhole
Example: Cygnus X-1
If the core of a collapsing star has a mass that is greater than three Suns, no known force can prevent it from forming a black hole. If a black hole has a nearby companion star, gas pulled away from the companion will be heated to tens of millions of degrees, producing X-rays as it falls toward the black hole. Radiation from the hot gas can be detected until the gas passes beyond the event horizon of the black hole. The spectrum of Cygnus X-1 shows the effect of gravity on radiation from atoms about 70 miles from the event horizon.
Credit: NASA/CXC/SAO
Type1a Supernova
Example: Tycho's SNR
Subrahmanyan Chandrasekhar, the Chandra X-ray Observatory’s namesake, used relativity theory and quantum mechanics to show that if the mass of a white dwarf becomes greater than about 1.4 times the mass of the Sun—called the Chandrasekhar limit—it will collapse. If a white dwarf is a member of a binary star system, a nearby companion star could dump enough material onto the white dwarf to push it over the Chandrasekhar limit. The resulting collapse and explosion of the white dwarf are believed to be responsible for a Type Ia supernova – the type that produced Tycho’s supernova remnant.
Credit: NASA/CXC/Rutgers/J.Warren & J.Hughes et al.
Neutron Star
Example: Crab Nubula Pulsar
During a supernova, the core of a massive star can be compressed to form a rapidly rotating ball composed mostly of neutrons that is only twelve miles in diameter. A teaspoonful of such neutron-star matter would weigh more than one billion tons! Young, rapidly rotating neutron stars can produce beams of radiation from radio through gamma-ray energies. Like a rotating lighthouse beam, the radiation can be observed as a powerful, pulsing source of radiation, or pulsar, as in the case of the Crab Nebula pulsar shown here. The jets and rings are caused by high-energy particles flowing away from the pulsar.
Credit: NASA/CXC/MSFC/M.Weisskopf et al
Supernovas
Example: Type II / G292.0+1.8
A Type II supernova occurs when a massive star has used up its nuclear fuel and its core collapses to form either a neutron star or a black hole. Gravitational energy released by this process blows the rest of the star apart. The expanding stellar material produces shock waves that heat a multimillion-degree shell of gas that glows in X-rays for thousands of years. The supernova remnant G292.0+1.8 shown here has an estimated age of 1,600 years. The neutron star is the white dot below and to the left of center.
Credit: X-ray: NASA/CXC/Penn State/S.Park et al.; Optical: Pal.Obs. DSS
Plantary Nebula
Example: Cat's Eye (NGC 6543)
After the core-helium-burning giant phase, all of a Sun-like star’s available energy resources will be used up. The exhausted giant star will puff off its outer layer leaving behind a smaller, hot star with a surface temperature of about 50,000 degrees Celsius. When the high speed “stellar wind” from the hot star rams into the slowly moving material ejected earlier, the collision creates a complex and graceful filamentary shell called a planetary nebula. A composite image of the Cat’s Eye from Chandra (purple) and Hubble (red & green) shows where the hot, X-ray emitting gas appears in relation to the cooler material seen in optical wavelengths.
Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI
Blue Giant
Example: Eta Carinae
When a massive star uses up the hydrogen fuel in its central core, it expands enormously to become a red giant. Intense radiation from the blue giant pushes gas away at speeds in excess of 3 million miles per hour. The collision between the high speed “stellar wind” and the previously ejected red giant material creates a spectacular nebula. The massive star that has produced the nebula appears as the bright yellow dot near the center of this image, just outside the composite X-ray (blue)/optical (red & green) image.
Eta Carinae, one of the most luminous stars known in our galaxy, radiates energy at a rate that is 5 million times that of the Sun, and is estimated to have a mass of about 100 solar masses. The exact nature of Eta Carinae is unknown, but it may be an extreme example of a luminous blue variable. Such stars are violently unstable and likely explode as supernovas. The X-rays in this image may be caused by the collision of stellar winds rushing away from Eta Carinae and a suspected companion star.
Credit: X-ray: NASA/CXC/GSFC/M.Corcoran et al.; Optical: NASA/STScI
Type II Supernova
Example: G292.0+ 1.8
A Type II supernova occurs when a massive star has used up its nuclear fuel and its core collapses to form either a neutron star or a black hole. Gravitational energy released by this process blows the rest of the star apart. The expanding stellar material produces shock waves that heat a multimillion-degree shell of gas that glows in X-rays for thousands of years. The supernova remnant G292.0+1.8 shown here has an estimated age of 1,600 years. The neutron star is the white dot below and to the left of center.
Credit: X-ray: NASA/CXC/Penn State/S.Park et al.; Optical: Pal.Obs. DSS
Blue Supergiant
Example: Zeta Orionis
When compared to the Sun, the blue supergiant Zeta Orionis has 20 times the diameter, 30 times the mass, and 100,000 the total power output. The enormous power output of this star is driving the outer layers of its atmosphere away at speeds in excess of 4 million miles per hour. This wind speed is not steady, so rapidly moving groups of particles slam into slower ones, producing shock waves. These shock waves are the likely source of most of the X-rays from Zeta Orionis, though hot gas trapped in magnetic fields near the surface of the star may also produce X-rays.
Credit: NASA/EIT/W.Waldron, J.Cassinelli
Supershell
Example: Tarantula Nebula (30 Doradus)
The Tarantula Nebula is in one of the most active star-forming regions in the Local Group of galaxies to which the Milky Way belongs. Some massive stars in the Nebula are producing intense radiation and searing winds that carve out gigantic bubbles in the surrounding gas. Other massive stars have exploded as supernovas. The combined activity of many stellar winds and supernovas create expanding supershells that can trigger the collapse of clouds of dust and gas to form new generations of stars.
Credit: X-ray: NASA/CXC/PSU/L.Townsley et al.; Infrared: NASA/JPL/PSU/L.Townsley et al.
Supernova
Example: SN 2006gy
The top panel of this graphic is an artist's illustration that shows what SN 2006gy may have looked like if viewed at a close distance. The fireworks-like material in white shows the explosion of an extremely massive star. This debris is pushing back two lobes of cool, red gas that were expelled in a large eruption from the star before it exploded. The green, blue and yellow regions in these lobes shows where gas is being heated in a shock front as the explosion material crashes into it and pushes it backwards. Most of the optical light generated by the supernova is thought to come from debris that has been heated by radioactivity, but some likely comes from the shocked gas.
Credit: X-ray: NASA/CXC/UC Berkeley/N.Smith et al.
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