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For the First Time Hubble Directly Measures Mass of a Lone White Dwarf

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February 02, 2023 10:00AM (EST)Release ID: 2023-004
Diagram titled “Hubble measures deflection of starlight by a foreground object.” Diagram includes an illustration of the Hubble Space Telescope observing a distant star whose light curves around another object—a white dwarf star—in the foreground. The diagram shows the real position of the observed star, the light path of that star, and the apparent (“observed”) position of the star. The white dwarf is in the center of the diagram, on a grid that appears to be warped downward under the enormous mass of the white dwarf. Hubble is toward the lower right on the diagram, at the 5 o’clock position relative to the white dwarf. It is pointing up toward the 12 o’clock position, where the star appears to be (“observed star position”). A straight dashed line connects the observed star position with the telescope. The real star position is at 11 o’clock. A solid line curves from the real star position, around the white dwarf, to the telescope.

Summary

Astronomers Use a Trick of Nature to 'Weigh' a Dead Star

A famous 1923 short poem "Nothing Gold Can Stay" written by Robert Frost ends with: "So dawn goes down to day. Nothing gold can stay."

Though Frost was talking about a day on Earth, this also applies to the next 5 billion years, when the golden Sun will eventually burn out and collapse down to a seething white hot cinder of its former glory. Astronomers call the stellar remnant a white dwarf. An estimated 10 billion of these stellar corpses are scattered across our galactic graveyard.

Though the Sun's ultimate fate is far into the future, astronomers want to learn a lot about the white dwarfs now. White dwarfs give us clues into how stars evolve over billions of years. Astronomers collect a lot of information by dissecting the light from a white dwarf through spectroscopy. And, knowing a dwarf's mass is one of the most important factors in a star's evolution. But it's not easy to weigh a white dwarf, or any other kind of star. There aren't any bathroom scales in space.

Isaac Newton's law of universal gravitation allowed for Earth's mass to be estimated by watching the orbit of the Moon. The same Newtonian equations can be used to measure the mass of a white dwarf orbiting a companion star. But astronomers used Hubble to make a completely different mass estimate for a white dwarf that doesn’t have a stellar companion that's needed for applying Newtonian physics.

They had to turn to more modern physics – Einstein's general relativity — that explains how a massive object's gravity warps space. This creates a pothole in the fabric of space that bends the light of a background star as a foreground object is passing in front of it. The greater the deflection on the sky, the more massive the bypassing body. But the amount of deflection in infinitesimally small, and requires Hubble's sharp vision – and some patience. This precision measurement was done for the nearby, fast moving white dwarf LAWD 37. Astronomers measured a mass of 0.56 times our Sun's mass. And, this nicely agrees with theories about what a typical white dwarf should weigh.

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Full Article

Astronomers using NASA's Hubble Space Telescope have for the first time directly measured the mass of a single, isolated white dwarf — the surviving core of a burned-out Sun-like star. Researchers found that the white dwarf is 56 percent the mass of our Sun. This agrees with earlier theoretical predictions of the white dwarf's mass and corroborates current theories of how white dwarfs evolve as the end product of a typical star's evolution. The unique observation yields insights into theories of the structure and composition of white dwarfs. Until now, previous white dwarf mass measurements ...

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