A Dwarf (Helium) Flasher


White Dwarf ResurrectionPay no attention to the bright star in the center of the picture. The really interesting stellar object in the frame is that blob of red near the bottom. It’s a small white dwarf undergoing a helium flash.

Normally, the white dwarf stage is the end of the life cycle of a low-mass star, but in some rare cases, a star reignites in a helium flash and expands to its previous red giant state. When this happens, huge amounts of gas and dust are ejected before the star shrinks to become a white dwarf again.

A helium flash is a dramatic and short-lived series of events, and this star—Sakurai’s Object, named for the Japanese amateur astronomer who discovered it—has allowed astronomers a rare opportunity to study a helium flash as it occurred.

Image Credit: ESO

Weighing a Dwarf


Sirius A&BAccurately determining the masses of white dwarfs is important to understanding stellar evolution. The Sun will eventually become a white dwarf. White dwarfs are also the source of Type Ia supernova explosions. The method used to calculate a white dwarf’s mass relies on a prediction from Einstein’s theory of General Relativity—that light loses energy when it attempts to escape the gravity of a compact star. This effect is known as the gravitational redshift.

The Hubble image above shows Sirius, the brightest star in our nighttime sky. The bright blob in the center is Sirius A. The dim spot at around 7 o’clock is A’s white dwarf companion star Sirius B. The image of Sirius A was overexposed so that dim Sirius B  could be seen. The X-shaped diffraction spikes and concentric rings around Sirius A and the small ring around Sirius B are artifacts produced within Hubble‘s optical and imaging systems. The two stars revolve around each other every 50 years. Sirius is only 8.6 light-years from Earth, making it the fifth closest star system known.

White dwarfs are the leftover remnants of stars similar to the Sun. They have exhausted their nuclear fuel sources and have collapsed down to a very small size. Sirius B is about 10,000 times dimmer than Sirius A. It’s very faint because of its tiny size, only about 12,000 km in diameter. Sirius B’s weaker light makes it a challenge to study, because its light is swamped in the glare of its brighter companion as seen from telescopes on Earth. Hubble‘s Imaging Spectrograph was able to isolate the light from Sirius B and was able to resolve light from Sirius B being stretched to longer, red-shiftedr wavelengths by the white dwarf’s gravity. Based on those measurements, astronomers have calculated Sirius B’s mass at roughly 98 percent that of the Sun. Further analysis of the star’s spectrum showed that its surface temperature is about 25,000 K.

Image Credit: NASA

A Zombie Star


zombieA team of astronomers using the Hubble Space Telescope has found a star system that may have left behind a “zombie star” after an unusually weak supernova explosion. A supernova normally obliterates the exploding white dwarf, and the star effectively dies. Scientists believe this faint supernova may have left behind a surviving portion of the dwarf star—a sort of zombie star. The two inset images show before-and-after images captured by Hubble of Supernova 2012Z in the spiral galaxy NGC 1309. The white X at the top of the main image marks the location of the supernova in the galaxy.

Image Credit: NASA

A Dwarf Flasher


White Dwarf ResurrectionPay no attention to the bright star in the center of the picture. The really interesting stellar object in the frame is that blob of red near the bottom. It’s a small white dwarf undergoing a helium flash.

Normally, the white dwarf stage is the end of the life cycle of a low-mass star, but in some rare cases, a star reignites in a helium flash and expands to its previous red giant state. When this happens, huge amounts of gas and dust are ejected before the star shrinks to become a white dwarf again.

A helium flash is a dramatic and short-lived series of events, and this star—Sakurai’s Object, named for the Japanese amateur astronomer who discovered it in 1996—has allowed astronomers a rare opportunity to study a helium flash as it occurred.

Image Credit: ESO

IC 1295


The glowing green planetary nebula IC 1295 surrounds a dim and dying star. It is located about 3300 light-years away in the constellation of Scutum (The Shield). The white dwarf star is softly shedding its outer layers, like an unfolding flower in space. It will continue this process for a few tens of thousands of years.

Image Credit: ESO

NGC 2440


Hubble reveals NGC 2440This Hubble Space Telescope image shows the colorful end of a star like the Sun. This star is casting off its outer layers of gas, which formed a cocoon around the star’s remaining core. Ultraviolet light from the dying star makes the material glow. The burned-out star, a white dwarf, is the white dot in the center. Our Sun will eventually burn out and surround itself with stellar debris, but that’s not expected for another 5 billion years or so.

The galaxy is filled with these stellar relics called planetary nebulae. (They have nothing to do with planets. 18th- and 19th-century astronomers used that name because through small telescopes the nebulae resembled the disks of the planets Uranus and Neptune.) This planetary nebula in this image is named NGC 2440. The white dwarf at the center of NGC 2440 is one of the hottest known, with a surface temperature of more than 200,000°C. The nebula’s chaotic structure suggests that the star shed its mass in multiple stages. During each outburst, the star blew off material in a different direction, resulting in the two bowtie-shaped lobes.

The material expelled by the star glows with different colors depending on its composition, its density and how close it is to the hot central star. Blue samples helium; blue-green oxygen, and red nitrogen and hydrogen.

Image Credit: NASA / ESA

The Case of the Hungry Dwarf


disintegrating_asteroid smallThe K2 mission using the repurposed Kepler Space Telescope has uncovered strong evidence of a tiny, rocky object being torn apart as it spirals around a white dwarf star. (That’s an artist’s concept of it above.) This discovery supports the theory that a white dwarf is capable of cannibalizing any planets that have survived within its solar system. As stars like our Sun age, they first swell into red giants and then gradually lose about half their mass, shrinking down to 1/100th of their original size (roughly the size of Earth). This dense star remnant is called a white dwarf.

During a series of observations last year, K2 was used to measure the minuscule change in brightness of  distant white dwarf known as. WD 1145+017. When an object passes in front of a star from our vantage point, a dip in the starlight can be detected. A periodic dimming indicates the presence of an object in orbit around the star. The object found around WD 1145+017 was the first found orbiting a white dwarf. It’s orbital period is only 4.5 hours, placing extremely close to the white dwarf and its searing heat and shearing gravitational force.

comparetotransit2While there was a prominent dip in brightness every 4.5 hours, the transit signal of the tiny planet did not exhibit a normal symmetric U-shaped pattern. The data (shown as the black dots) displayed an asymmetric elongated slope pattern that indicated the presence of a comet-like tail. Together these features indicated a ring of dusty debris circling the white dwarf. That could be the signature of the small planet being vaporized.

In addition to the strangely shaped transits, other data shows signs of heavier elements polluting the atmosphere of WD 1145+017. Because of their intense gravity, white dwarfs are expected to have chemically pure surfaces and be covered only with the light elements helium and hydrogen. For years, researchers have found evidence that some white dwarf atmospheres are polluted with traces of heavier elements such as calcium, silicon, magnesium, and iron, and scientists have long suspected that the source of such pollution could come from a planet being torn apart. It looks as if they found one.

Image Credits: CfA / A. Vandenberg

Weighing Sirius B


Sirius A&BAccurately determining the masses of white dwarfs is important to understanding stellar evolution. The Sun will eventually become a white dwarf. White dwarfs are also the source of Type Ia supernova explosions. The method used to calculate a white dwarf’s mass relies on a prediction from Einstein’s theory of General Relativity—that light loses energy when it attempts to escape the gravity of a compact star. This effect is known as the gravitational redshift.

The Hubble image above shows Sirius, the brightest star in our nighttime sky. The bright blob in the center is Sirius A. The dim spot at around 7 o’clock is A’s white dwarf companion star Sirius B. The image of Sirius A was overexposed so that dim Sirius B  could be seen. The X-shaped diffraction spikes and concentric rings around Sirius A and the small ring around Sirius B are artifacts produced within Hubble‘s optical and imaging systems. The two stars revolve around each other every 50 years. Sirius is only 8.6 light-years from Earth, making it the fifth closest star system known.

White dwarfs are the leftover remnants of stars similar to the Sun. They have exhausted their nuclear fuel sources and have collapsed down to a very small size. Sirius B is about 10,000 times dimmer than Sirius A. It’s very faint because of its tiny size, only about 12,000 km in diameter. Sirius B’s weaker light makes it a challenge to study, because its light is swamped in the glare of its brighter companion as seen from telescopes on Earth. Hubble‘s Imaging Spectrograph was able to isolate the light from Sirius B and was able to resolve light from Sirius B being stretched to longer, red-shiftedr wavelengths by the white dwarf’s gravity. Based on those measurements, astronomers have calculated Sirius B’s mass at roughly 98 percent that of the Sun. Further analysis of the star’s spectrum showed that its surface temperature is about 25,000 K.

Image Credit: NASA

A Dwarf Flasher


White Dwarf ResurrectionPay no attention to the bright star in the center of the picture. The really interesting stellar object in the frame is that blob of red near the bottom. It’s a small white dwarf undergoing a helium flash.

Normally, the white dwarf stage is the end of the life cycle of a low-mass star, but in some rare cases, a star reignites in a helium flash and expands to its previous red giant state. When this happens, huge amounts of gas and dust are ejected before the star shrinks to become a white dwarf again.

A helium flash is a dramatic and short-lived series of events, and this star—Sakurai’s Object, named for the Japanese amateur astronomer who discovered it in 1996—has allowed astronomers a rare opportunity to study a helium flash as it occurred.

Image Credit: ESO

A Zombie Star


zombieA team of astronomers using the Hubble Space Telescope has found a star system that may have left behind a “zombie star” after an unusually weak supernova explosion. A supernova normally obliterates the exploding white dwarf, and the star effectively dies. Scientists believe this faint supernova may have left behind a surviving portion of the dwarf star—a sort of zombie star. The two inset images show before-and-after images captured by Hubble of Supernova 2012Z in the spiral galaxy NGC 1309. The white X at the top of the main image marks the location of the supernova in the galaxy.

Image Credit: NASA

The Helix Nebula


helixnebulaThis infrared image taken by the Spitzer Space Telescope looks a bit like Sauron’s eye. It’s the Helix Nebula, a cosmic site often photographed by amateur astronomers because of its vivid colors and eerie resemblance to a giant eye. It’s about 700 light-years away in the constellation Aquarius and belongs to a class of objects called planetary nebulae.

Planetary nebulae are the remains of stars that were once like our Sun. When these stars die, they puff out their outer gaseous layers which are heated by the hot core of the dead star. The remnant becomes a white dwarf and shines with infrared and visible light. Our Sun probably will expand into a planetary nebula in around five billion years.

Spitzer‘s infrared view of the Helix nebula shows the outer gaseous layers is in blues and greens. The red color in the middle of the eye is the final layers of gas blown out when the star died. Blue shows infrared light of 3.6 to 4.5 µm wavelengths, green shows infrared light of 5.8 to 8 µm, and red shows infrared light of 24 µm.

The brighter red circle in the very center is the glow of the dust circling the white dwarf. This dust is thought to have been kicked up by comets that survived the death of the original star. Before the star died, its comets and possibly planets would have orbited the star in an orderly fashion. But when the star blew off its outer layers, it’s inner planets would have been swallowed up in its expanding shell, but the icy bodies and outer planets would have been stirred up and into tossed into each other, creating a cosmic dust storm. The Helix nebula is one of only a few dead-star systems in which evidence for cometary survivors has been found.

Image Credit: NASA