Focus on Gliese 433.1 -- Have You Seen a White Dwarf?


For telescopes 4" or larger

Many years ago I dreamed of one day detecting the companion to Sirius.  Sirius is one of the closest stars (8.6 ly) and is orbited by the first star recognized to be a white dwarf.  A fine instrument and a fine night are necessary to observe the companion visually, even though at 8.5 magnitude it is bright enough to be seen in binoculars.  Unfortunately, the glare from the nearby -1.4 magnitude Sirius all but extinguishes it.

The other factor working against me has been timing.  It takes 50 years for the companion to make one apparent orbit about Sirius.  As seen from earth this orbit is elliptical, taking it from a maximum separation of 11" in 1971 to a minimum of 2.5" in the early 90's.  The stars are currently separated by about 6" and will continue to widen over the next 20 years.  As they separate it becomes less difficult to spot the companion.  Hopefully I will finally get my chance to bag this elusive target sometime in the years to come.

I haven't given up on the companion to Sirius, but there are other white dwarfs that can be observed visually.  Gliese 433.1 presents an excellent opportunity for observers to bag a white dwarf in just about any telescope.  Unlike the companion to Sirius, Gliese 433.1 is a lone star.  It lies relatively nearby at a distance of 50 light years and shines at magnitude 12.5.

Gliese 433.1 (yellow cross hairs) is found in far southern Ursa Major, nearly in Leo.


The field in a 90mm ETX at 39x. The field of view (circle) is 103'.

Don't expect to see much in the eyepiece.  Gliese 433.1 will appear like any other star in the field.  It will appear as a relatively faint, colorless star even in large aperture instruments.   But like so many objects in the night sky it is knowing that you are looking at something special that brings satisfaction.   Finding this star may be a bit of a challenge in telescopes smaller than 6 inches; good finder charts are a must.

When you gaze upon Gliese 433.1, consider that you are viewing the still warm corpse of a once proud star that shown brilliantly, perhaps in the sky of a planet.  Consider that it once stood perhaps 200,000 times larger and shone 10,000 times brighter than today.  Consider that it is made of a bizarre material unlike anything known here on earth, material so dense that one teaspoonful on earth would weigh as much as a Mack truck.  

WHITE DWARFS ARE A DEGENERATE MATTER


A white dwarf is the end product of a typical star (up to around 10 times the mass of our sun).   The life of any star can be viewed as the inevitable collapse of a cloud of gas and dust.  As this cloud collapses, perhaps after the passing shock of a supernova explosion, the gas is compressed into an ever smaller body.  The combined mass of this gas can be quite large and as it compresses the pull of gravity strengthens, causing it to collapse even further, which in turn strengthens the gravity, compressing it even further, and so on.  A ball of ever more compressed gas is formed.

As you compress a gas it becomes hotter.  Eventually you reach a state at the very center of the collapsing ball of gas where the combination of great compression and heat causes protons to fuse together, beginning a nuclear a reaction that builds Helium and produces energy.  A single proton is the nucleus of the most simple and abundant element, Hydrogen, so we call this process Hydrogen fusion.

All the energy now being produced at the center of the collapsing ball of gas has to be released.  It travels outward through the gas ball producing an outward pressure.  This outward pressure offsets the pull of gravity and eventually a balance is reached and the ball stops collapsing.  We call this stable ball of glowing gas a star.

The one overriding factor that describes the life of a star is how massive it is.  If very massive, the strength of gravity is very high and a great deal of energy must be produced to keep it from collapsing.  These stars must "burn" their fuel at an enormous rate.  They are very bright and very hot and they can only keep this up for a short time before they begin to run short of Hydrogen to fuse at their centers.

The stars that began with less gas make smaller fainter cooler stars that can sustain their fusion for a very long time.  Regardless of how massive the star is, eventually Helium will collect at the center and Hydrogen will become scarce.  The result is a further collapse and less energy is generated.  What collapses is primarily the "core" of the star where the Helium has collected.  This collapse produces heat and draws Hydrogen toward the center, which eventually allows more energy to be produced.  In fact, too much energy is produced, the result of which is that the outward pressure of the flowing energy now overcomes the inward pressure of gravity and the outer layers of the star swell.  The star becomes very large and to outward appearances cooler, thus redder.  We call this stage of the life of a star a red giant.

Eventually Helium can also be fused in the core of the star and the process sort of repeats itself.  Depending on the mass of the star other elements may be fused, each requiring higher temperature and pressure.  Eventually there will be nothing left to fuse and without the outward energy flow to hold it up the collapse which began so long ago will continue.  What will stop it now?

In the process of being a red giant the star can literally blow much of its self away.  Astronomers call this "mass loss."   If after this mass has been shed the star still contains more than 1.4 times the mass of our sun a dramatic supernova explosion will occur.  But for the vast majority of stars this will never happen.  Instead, the star will continue to collapse and change.  After reaching a density of nearly a million times that of water, matter will become so compressed that electrons are squeezed closely together; so closely that their natural repulsion will act as a force to finally halt the collapse.  Astronomers call this form of highly compressed matter "degenerate".

The remaining star, called a white dwarf, will have shrunk to the size of our tiny earth!  It is made primarily of oxygen and carbon, the heaviest byproducts of the fusion processes.  If a star is like a burning fire, then a white dwarf is like a hot ember.  Its energy comes not from fusion, but is merely left over heat from the collapse.  Eventually a white dwarf will cool and fade away forever, becoming dark.  The matter of the star will "solidify" and the carbon will crystallize to form something very similar to an enormous diamond!

 

R.A. Dec. Con Mag Distance
Gliese 433.1 aka HIP 56662 11h37m05.1 +29o47'58" UMa 12.5 50 ly

Millennium Star Atlas Vol II Chart 657
Sky Atlas 2000 Chart 6
Uranometria 2000 Vol I Chart 106
Herald-Bobroff Astroatlas B-09 C-30

Color image note: this image was created using red and blue second generation DSS images. The field of view is one degree. North is down and east is to the right.
 

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