White Dwarfs
Overview
A white dwarf is a compact remnant—the final evolutionary state of over 95% of the stars in the Milky Way. This fate is determined by the mass of the star. Stars within the range of 0.08 to 8 solar masses become a red giant, shed their layers in a planetary nebula, ultimately revealing a compact core called a white dwarf. Stars less massive than 0.08 solar masses have long lifetimes on the order of trillions of years. Stars more massive than 8 solar masses will result in either a neutron star or black hole.
An Hubble Space Telescope image of the star Sirius, and it’s faint companion white dwarf, Sirius B, located to the lower-left of Sirius. Sirius has prominent diffraction spikes resulting from the optics of the telescope. Credit: NASA, ESA, H. Bond (STScI), and M. Barstow (University of Leicester)
Extraordinarily Dense
The size of a white dwarf is approximately that of Earth, but its mass is on the order of the Sun’s mass. So, they are extremely dense objects—one billion kilograms per cubic meter. Astronomers call the material that makes up a white dwarf degenerate matter. Inside a normal star, gravitational forces pulling inward are balanced by the internal pressure produced from the release of energy. However, inside a white dwarf, there is no internal pressure to counter gravity, so the object is further compacted by the force of gravity until the electrons of neighboring atoms are smashed up against one another. The object cannot collapse further because, atomically, the electrons have nowhere to go.
Ultimate Fate
The Sun will become a white dwarf. It will transition into a red giant in about six billion years and will expand to envelope Earth in about 7.6 billion years. Within about 200 million years after that, the Sun will shed its outer layers into a planetary nebula, and a white dwarf will emerge as the Sun’s core remnant.
Over time, a white dwarf will cool slowly and eventually fade from view, ultimately becoming what is known as a black dwarf. The timeline for this gradual fade; however, is longer than the age of the universe. So, at this point in cosmic history, no black dwarfs yet exist.
Representing White Dwarfs
Note
We represent the white dwarfs with generic points with exaggerated brightness. Larger points indicate the object is closer to you.
White dwarfs are too dim to see with the unaided eye. The first one observed, called 40 Eridani B, was discovered by the famous astronomer William Herschel (1738–1822) in 1783. The second one was discovered in 1862 as a companion to the star Sirius.
Because these are dim objects, we exaggerate their appearance here by representing them with points far brighter than their actual brightness. The relative size of the points describes their proximity to you, rather than the object’s intrinsic brightness—so the larger the point, the closer it is to you. This exaggeration allows us to see where they are in the sky, and how they’re distributed around the Sun. But, it’s essential to remember all of these objects are invisible to us in the night sky.
A night-sky view of the white dwarfs looking toward Orion. We see there is no recognizable night sky. The constellation lines are drawn merely to provide some reference toward where we’re looking. Two well known white dwarfs are visible in this image: Sirius B is located where Sirius would normally be in Canis Major, and 40 Eridani B, the brightest object located directly to the right of Sirius B.
A view of the night sky looking toward Scorpius and Sagittarius but, instead of seeing the stars we see the white dwarfs. The constellation lines are on for reference.
Distribution in the Galaxy
Because white dwarfs gradually cool over many billions of years, we expect to see them wherever we see stars. They are spherically distributed around the Sun, not limited to more recent stellar activity in the disk of the Galaxy.
This spherical huddle around the Sun is, of course, limited by what we can see—these data, like all the data here, are a product of our observations and those observations happen from Earth (or low-Earth orbit, which is essentially the same).
The distribution of white dwarfs in the Milky Way. These objects are huddled close to the Sun, which lies at the center of the cluster of white dwarf points. The Galaxy is, of course, filled with these objects, but they are too faint to see at great distances from the Sun. The lines jutting out from the white dwarf data are the constellation lines, represented in 3-D from this vantage point.
Profiles
Dossier
Census: |
192,613 white dwarfs |
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OpenSpace Version: |
1 |
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Prepared by: |
Zack Reeves, Brian Abbott (AMNH) |
Source Version: |
1.01 |
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