- 3863 - WHITE DWARF - measuring the mass? How was microlensing used to measure the mass of a white dwarf star? Astronomers have directly measured the mass of a dead star using an effect known as “gravitational microlensing”.
--------------- 3863 - WHITE DWARF - measuring the mass?
- The
international team, led by the University of Cambridge, used data from two
telescopes to measure how light from a distant star bent around a white
dwarf “LAWD 37”, causing the distant
star to temporarily change its apparent position in the sky.
-
- This is the
first time this effect has been detected for a single, isolated star other than
our Sun, and the first time the mass of such a star has been directly measured.
-
- LAWD 37 is
a white dwarf, the result of the death of a star like our own. When a star
dies, it stops burning its fuel and expels its outer material, leaving only a
hot, dense core. Under these conditions, matter as we know it behaves very differently
and turns into something called “electron-degenerate matter”.
-
- White
dwarfs give us clues into how stars evolve, someday our own star will end up as
a white dwarf. This white dwarf is 15
light-years away in the Musca constellation and is what remains of a star that
died 1.15 billion years ago.
-
- Because
this white dwarf is relatively close to us, we’ve got lots of data on it. We’ve got information about its spectrum of
light, but the missing piece of the puzzle has been a measurement of its mass.
-
- Mass is one
of the most important factors in a star’s evolution. For most stellar objects,
astronomers infer mass indirectly. In rare cases where mass can be directly
inferred, the object has to have a companion, such as a binary star system. But
for single objects, such as LAWD 37, other methods for determining mass are
needed.
-
- In his
General Theory of Relativity, Einstein predicted that when a massive compact
object passes in front of a distant star, the light from the star would bend
around the foreground object due to its gravitational field.
-
- This effect
is known as “gravitational microlensing”. In 1919, two British astronomers –
Arthur Eddington from Cambridge and Frank Dyson from the Royal Greenwich
Observatory – first detected this effect during a solar eclipse, in what was
the first popular confirmation of General Relativity. However, Einstein was
pessimistic that the effect would ever be detected for stars outside our solar
system.
-
- In 2017,
astronomers detected this gravitational microlensing effect for another nearby
white dwarf in a binary system, “Stein 2051 b”, which marked the first
detection of this effect for a star other than our Sun. Now, the Cambridge-led
team has detected the effect for LAWD 37, giving the first direct mass
measurement for a single white dwarf.
-
- Using ESA’s
Gaia satellite, which is creating the most accurate and complete
multi-dimensional map of the Milky Way, the astronomers were able to predict
the movement of LAWD 37 and identify the point where it would align close
enough to a background star to detect the lensing signal.
-
- Using the
Gaia data, the astronomers were able to point The Hubble Space Telescope in the
right place at the right time to observe this phenomenon, which happened in
November 2019, 100 years after the famous Eddington/Dyson experiment.
-
- Since the
light from the background star was so faint, the main challenge for astronomers
was extracting the lensing signal from the noise. These events are rare, and the effects are
tiny. For instance, the size of our
measured effect is like measuring the length of a car on the Moon as seen from
Earth, and is 625 times smaller than the effect measured at the 1919 solar eclipse.
-
- Once they
had extracted the lensing signal, the researchers were able to measure the size
of the astrometric deflection of the background source, which scales with the
mass of the white dwarf, and obtain a gravitational mass for LAWD 37 that is 56
percent the mass of our Sun.
-
- This agrees
with earlier theoretical predictions of LAWD 37’s mass, and corroborates
current theories of how white dwarfs evolve.
The precision of LAWD 37’s mass measurement allows us to test the
mass-radius relationship for white dwarfs, testing the properties of matter
under the extreme conditions inside this dead star.
-
February 5, 2023 WHITE
DWARF - measuring the mass? 3863
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--- Tuesday, February 7, 2023 ---------------------------
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