Star Types
White Dwarfs
You should know -
♣ The White Dwarf is also know as a Degenerate Dwarf due to the fact that it is composed mostly of electron-degenerate matter. This star type was first discovered in 1844 but only indirectly. The telescopes of that time were unable to directly view the star, the faint companion of Sirius, and therefore it's presence was inferred by the noticeable wavy motion in it's binary partner as the two revolved around each other
♣ Degrees Kelvin - 1 Kelvin = -457.87 degrees fahrenheit or 1 degree fahrenheit = 255.9277778 Kelvin
♣ Cataclysmic Variable Star - Stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state
♣ Variable Star of RR Lyrae Type - Old, relatively low mass variable stars often used as standard candles
♣ Standard Candles - A standard candle is a class of astrophysical objects, such as supernovae or variable stars, which have known luminosity due to some characteristic quality possessed by the entire class of objects
♣ Wolf-Rayet or WR Star - An evolved, massive stars (over 20 solar masses), which are losing mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s
The Stellar Gem - Going Out in Style
Source: NASA/R. Ciardullo (PSU)/H. Bond (STScI)
Seen above is the planetary Nebula NGC2440 with its very hot central star, white dwarf HD 62166. The dwarf has an exceptionally high surface temperature of 200,000 kelvins (359,540.33 degrees f ), one of the hottest known.
Having nearly spent it's nuclear fuel of hydrogen, a change in the star's mass is about to take place. Shedding it's outer layers into fast moving stellar winds (which in turn form a planetary nebula) the star is now reducing itself in size while becoming much more dense at it's core. In fact, the density of the White Dwarf is going to become so dense that it will only be outdone by the density of a neutron star. What remains is the star's core, an object little bigger than our own planet. It is still a very hot star at around 100,000 Kelvin ( 179,540.33° f ) but the engine that has kept this star going is slowly burning out having consumed all available resources and now begins to cool down during it's last billion years or so of life. See: White Dwarfs Ask an Astrophysicist at the Nasa website.
You may have noticed that I used the term "stellar gem" above? Well, it was used quite literally. You see, a White Dwarf can be compared to a diamond in that it's underlying inner core is so dense that even carbon is put under a fantastic amount of pressure. It is thought that this area of the white dwarf is composed of a crystalline lattice of carbon and oxygen atoms. And since a diamond is itself crystallized carbon...well, you see the point. If you must go, do it in style!
Although hidden inside the core of a white dwarf, a cutaway view of the diamond star would show quite a sparkle! Source Credit: Travis Metcalfe, Christine Pulliam, and Ruth Bazinet, Harvard-Smithsonian Center for Astrophysics and CfA Article.
DENSITY - HEART OF THE MATTER
A Quick Aside
How dense is a white dwarf star? Well, to answer that in everyday terms we'll need some water please. You see, the standard for measuring the density of something was established using good old H2O which in this case is about 62 lbs per cubic foot, compared to dry air which is only .07967 pounds per cubic foot. Of course, other variables affect this standard but we'll keep it simple.
First, when we speak about the "Density" of something what we're referring to are the particles that make up our element, gas or matter and just how tightly those particles are packed or compressed together.
Now, taking our water, we can put an ice cube into it and it will float however, try putting a piece of carbon the same size as the ice cube into the water and what happens? Yep, right to the bottom of the glass and...oops...crack! That's because carbon has a density of about 134 lbs per cubic foot. After carbon is compressed to form a diamond it achieves a density of 219.24 pounds per cubic foot. The pressure required to do that is equal to the Eiffel Tower turned upside down, with all its weight resting on a plate 5 inches square or 4,000,000 lbs psi. For a diamond to form inside our star it has to reach a density equal to 10(^6) times that of water or 62,000,000 pounds per cubic foot. Put in anther way and remebering how some stars with "super gravity" used to be described: since there is about 5745 teaspoons per cubic foot, we divide that by 62,000,000 and you get a weight per teaspoon of White Dwarf star material of 10,791 pounds - and that's how dense it is.
"A hundred million years after the red giant phase all of the star's available energy resources will be used up. The exhausted red giant will puff off its outer layer leaving behind a hot core. This hot core is called a Wolf-Rayet type star after the astronomers who first identified these objects. This star has a surface temperature of about 50,000 degrees Celsius and is furiously boiling off its outer layers in a "fast" wind traveling 6 million kilometers per hour.
The radiation from the hot star heats the slowly moving red giant atmosphere and creates a complex and graceful filamentary shell called a planetary nebula (so called because it looks like the disk of a planet when viewed with a small telescope). X-ray images reveal clouds of multimillion degree gas that have been compressed and heated by the fast stellar wind. Eventually the central star will collapse to form a white dwarf star." Source Reference: Chandra Website White Dwarfs & Planetary Nebulas
The White Dwarf, Artist Rendering
Birth of a White Dwarf Star. Click on image to enlarge.
An Elusive Quarry
Searching the globular cluster known as M4 with the Hubble Space Telescope's Wide Field Planetary Camera 2, a trio of objects is revealed: "The Planet, the White Dwarf, and the Neutron Star" with the white dwarf the most visible of of the three (arrow). White Dwarfs have proven themselvevs to be a most enduring if elusive stellar object, one that was first observed in the middle of the 19th century. I say elusive (if the above image itself doesn't project that feeling) diue to the fact that they are amonst the dimmest stars in the universe; enduring becuase these stars have commanded a large amount of intrest due, in no small part, to the fact that they are what will eventually become of our own star, the sun, in 5-6 billion years or so. Source Reference: APOD To view full sized image Click Here
Having a surface tempeture of 35,500 kelvin (63,440.33 degrees f ) the above image shows IK Pegasi B and it's size relationship between its A-class companion IK Pegasi A (left) and the Sun (right) and may also help illustrate why the white dwarf can sometimes prove to be a hard star to locate. IK Pegasi A is currently considered a main sequence star that is right on the edge due to it's placement on H-R chart known as the instability strip. Stars in this band oscillate in a coherent manner, resulting in periodic pulsations in the star's luminosity. Source Reference: ESO Report...IK Pegasi (HR 8210) and IK Pegasi Wikipedia entry.
| Star Name & Type | S-type | ICRS location Ra & Dec |
| BD+18 4794B IK Peg Spectroscopic binary | A8m (C) | 21 26 26.6624 +19 22 32.304 |
Reference: Simbad, C.D.S. - SIMBAD4 rel 1.101 - 2008.11.04CET01:48:18
Of course optical viewing is not the only method employed when looking for white dwarfs. Above is HZ 43, also known as NSV 6165, which was observed by the X-ray satellite ROSAT.
Click on image to jump to the ROSAT directory at NASA/GODDARD
| Star Name & Type | S-type | ICRS location Ra & Dec |
| NSV 6165 White Dwarf | DAw | 13 16 21.8533 +29 05 55.440 |
Reference: Simbad, C.D.S. - SIMBAD4 rel 1.101 - 2008.11.04CET02:12:52
Above is both image and 10 arcmin plot around the white dwarf star NSV 6165 as seen in the x-ray image previous. The plot is the result of a query via the SIMBAD website and the RGB image was obtained through The Aladin Sky Atlas interface. The information is first queryed via Simbad and then used to create a plot containing 2 overlays - a digitized graphic image and a plot with symbols (removed for clarity) representing various stellar objects found around the main query subject.
Again we take a deep look into star cluster M4 with Hubble's Wide Field Planetary Camera 2 and we find quite a few more white dwarf stars. The reason for looking into globular star clusters for white dwarfs is that these types of star groups are very, very old. Since a white dwarf is a star that is approaching the end of it's life it goes without saying that we would search where stellar life is oldest. Studying how these stars cool could lead to a better understanding of their ages, of the age of their parent globular cluster, and even the age of our universe. Source Reference: APOD
| Star Name & Type | S-type | ICRS location Ra & Dec |
| M4 Globular Cluster, NGC6121 | F8 | 16 23 35.41 -26 31 31.90 |
And where do we find White Dwarfs?
That really depends on where you look. In answer to that, and keeping in mind that we want to look where stellar life is oldest, I plotted an area just 10 arcmin square, centered on the Glodular Cluster M4 and the results were a listing of 5,255 stellar objects. After subtracting those that were listed as a "Star in Cluster" I came up with the following identified White Dwarf stars:
| Star Name | S-type | Location (ra & dec) |
| WD J1623-266 White Dwarf | (~) | 16 23 38.232 -26 31 53.36 |
| [KRH2004] WD 1 White Dwarf | (~) | 16 23 52.76 -26 33 13.1 |
| [KRH2004] WD 2 White Dwarf | (~) | 16 23 54.49 -26 31 19.4 |
| [KRH2004] WD 12 White Dwarf | (~) | 16 23 53.25 -26 33 10.7 |
| Cl* NGC 6121 IRF 4430 Pulsating White Dwarf | (~) | 16 23 54.12 -26 33 29.0 |
| [KRH2004] WD 3 White Dwarf | (~) | 16 23 55.54 -26 32 36.6 |
| [KRH2004] WD 10 White Dwarf | (~) | 16 23 55.87 -26 32 37.7 |
| [KRH2004] WD 4 White Dwarf | (~) | 16 23 55.87 -26 33 03.3 |
| [KRH2004] WD 11 White Dwarf | (~) | 16 23 58.12 -26 32 10.1 |
| Cl* NGC 6121 IRF 3803 Pulsating White Dwarf | (~) | 16 23 58.88 -26 32 00.0 |
| Cl* NGC 6121 IRF 4595 Pulsating White Dwarf | (~) | 16 23 56.72 -26 33 48.3 |
| [KRH2004] WD 5 White Dwarf | (~) | 16 23 58.75 -26 32 40.1 |
| [KRH2004] WD 13 White Dwarf | (~) | 16 23 59.35 -26 31 28.5 |
| [KRH2004] WD 6 White Dwarf | (~) | 16 24 02.33 -26 32 25.6 |
| [KRH2004] WD 7 White Dwarf | (~) | 16 24 02.81 -26 32 42.6 |
| [KRH2004] WD 8 White Dwarf | (~) | 16 24 03.10 -26 32 45.1 |
| [CR86] 831 White Dwarf | (~) | 16 24 10 -26 33.2 |
It is also worth noting the many different types of stellar objects within this globular cluster
| PSR B1620-26 b Extra-solar Planet Candidate |
| PSR J1623-2631 Pulsar |
| SV BV 576 Variable Star |
| [BPH2004] CX 4 Cataclysmic Variable Star |
| [BPH2004] CX 11 X-ray source |
| IRAS 16205-2626 Infra-Red source |
| DoAr 14 Emission-line Star |
| CCDM J16234-2622B Star in double system |
| V* V972 Sco Variable Star of delta Sct type. |
Source Reference: Simbad, C.D.S. - SIMBAD4 rel 1.101 - 2008.11.04CET03:52:36
And for those that are curious ( as I was ), here is a plot, trimmed down to an area of just 5 arcmin around the center of M4:
The largest concentration of stars occur in this area, denoted by overlapping red circles.
A White Dwarf's Finale - Bing, Bang, Boom!
After stars like the Sun fuse their last atom, they scatter their gas and dust into space as a planetary nebula, such as Abell 39. The above image was taken at the WIYN Observatory's 3.5-m (138-inch) telescope at Kitt Peak National Observatory, Tucson, AZ, in 1997 through a blue-green filter that isolates the light emitted by oxygen atoms in the nebula at a wavelength of 500.7 nanometers. The nebula has a diameter of about five light-years, and the thickness of the spherical shell is about a third of a light-year. The nebula itself is roughly 7,000 light-years from Earth in the constellation Hercules. Source Credit NOAO Website
| Star Name & Type | S-type | ICRS location Ra & Dec |
| Abell 39, A39, ACO 39(180) or ARO 180 | (~) | 00 28 20 -11 23.4 & PNG 047.0+42.4, PK 047+42.1 |
Reference: Simbad, C.D.S. - SIMBAD4 rel 1.101 - 2008.11.04CET02:12:52
"The dramatic death of a white dwarf star in a violent explosion has been simulated on computers for the first time. Because distant exploding stars are used to track the expansion rate of the cosmos, astronomers say the feat could help in the quest for the ultimate fate of the universe." Source Credit: At last, virtual stars go kaboom on cue Article of 16:15 22 March 2007 NewScientist.com news service Hazel Muir. Image Credit: Lamb group/U of Chicago
Though we'll cover Supernova in another section, one aspect of the white dwarf is it's tendency to end in a spectacular thermonuclear explosion, one so great that it can be detected clear across the universe!
Supernova 2006gz, marked in this photo, shows the strongest evidence yet that it was caused by the merger of two white dwarfs. Source Credit: J.L. Prieto & M. Hicken (CfA) Harvard-Smithsonian Center for Astrophysics
| Star Name & Type | S-type | ICRS location Ra & Dec |
| SN 2006gz SuperNova | Fluxes - B 16.6 | 18 10 26.33 +30 59 44.4 |
Reference: Simbad, C.D.S. - SIMBAD4 rel 1.101 - 2008.11.04CET22:17:32
Location of Supernova 2006gz, seen on the outer edge of galaxy IC 1277, from the Aladin Sky Atlas. Source Credit: Aladin Website
| Star Name & Type | Morphological type | ICRS location Ra & Dec |
| IC 1277 Galaxy in Pair of Galaxies | Sc D | 18 10 27.33 +31 00 12.3 |
Colliding Dwarfs, Conclusion
Merging white dwarf stars...spectacular to say the least. The tremendous and fantastic meeting of two white dwarfs would certainly produce a little more "bang" than your ordinary supernova.
Well, in the case of 2000gz that is what just might have occured. Scientific measurements of that event showed the supernova to be of much greater strength than otherwise should have been when compared to a single white dwarf going supernova. Source Reference: NASA Credit for the above still images from the movie: NASA/Dana Berry, Sky Works Digital.
But this isn't the end of the story for a "scientist using NASA's Chandra X-ray Observatory has found that two white dwarf stars orbiting each other in a death grip, destined to merge spectacularly, may be flooding space right now with gravitational waves. These waves are ripples in space-time predicted by Einstein but never detected directly.
As a pair of white dwarfs steadily spiral inward, they churn the sea of space-time. The gravitational waves become more intense as the stars accelerate faster and faster, edging ever closer to a merger. Stars in the binary system RX J0806.3+1527 are only 50,000 miles apart. Merging white dwarfs might create a neutron star. Although they won't merge for another few hundred thousand years, these stars now might be one of the brightest sources of gravitational waves known. The Chandra X-ray Observatory has found indirect evidence for this; direct detection could come with the launch of the LISA mission." Credit Reference: NASA
Conclusion
As in all areas of astronomical research the White Dwarf has as much to share with us as any other object in our universe. The fact that the white dwarf is a star in it's last phases of stellar evolution doesn't mean the end to our study. The fact is the last hurrah of this type of star may only be the beginning of a much louder stellar roar or maybe a portent of things to come.
The last Hurrah of a Dying Star! Once again we look at the planetary nebula NGC2440 and it's central hot white dwarf star.
13-Feb-2007: "A brand new image taken with Hubble's Wide Field Planetary Camera 2 shows the planetary nebula NGC 2440 - the chaotic structure of the demise of a star." Source Reference:
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