Credits: NASA, ESA, and K. Sahu (STScI)
Release images
Astronomers have used the sharp vision of NASA’s Hubble Space Telescope to repeat a century-old test of Einstein’s general theory of relativity. The Hubble team measured the mass of a white dwarf, the burned-out remnant of a normal star, by seeing how much it deflects the light from a background star.
This observation represents the first time Hubble has witnessed this
type of effect created by a star. The data provide a solid estimate of
the white dwarf’s mass and yield insights into theories of the structure
and composition of the burned-out star.
First proposed in 1915, Einstein’s general relativity theory
describes how massive objects warp space, which we feel as gravity. The
theory was experimentally verified four years later when a team led by
British astronomer Sir Arthur Eddington measured how much the sun’s
gravity deflected the image of a background star as its light grazed the
sun during a solar eclipse, an effect called gravitational
microlensing.
Astronomers can use this effect to see magnified images of distant
galaxies or, at closer range, to measure tiny shifts in a star’s
apparent position on the sky. Researchers had to wait a century,
however, to build telescopes powerful enough to detect this
gravitational warping phenomenon caused by a star outside our solar
system. The amount of deflection is so small only the sharpness of
Hubble could measure it.
Hubble observed the nearby white dwarf star Stein 2051 B as it passed
in front of a background star. During the close alignment, the white
dwarf’s gravity bent the light from the distant star, making it appear
offset by about 2 milliarcseconds from its actual position. This
deviation is so small that it is equivalent to observing an ant crawl
across the surface of a quarter from 1,500 miles away.
Using the deflection measurement, the Hubble astronomers calculated
that the white dwarf’s mass is roughly 68 percent of the sun’s mass.
This result matches theoretical predictions.
The technique opens a window on a new method to determine a star’s
mass. Normally, if a star has a companion, astronomers can determine its
mass by measuring the double-star system’s orbital motion. Although
Stein 2051 B has a companion, a bright red dwarf, astronomers cannot
accurately measure its mass because the stars are too far apart. The
stars are at least 5 billion miles apart – almost twice Pluto’s present
distance from the sun.
“This microlensing method is a very independent and direct way to
determine the mass of a star,” explained lead researcher Kailash Sahu of
the Space Telescope Science Institute (STScI) in Baltimore, Maryland.
“It’s like placing the star on a scale: the deflection is analogous to
the movement of the needle on the scale.”
Sahu will present his team’s findings at 11:15 a.m. (EDT), June 7, at
the American Astronomical Society meeting in Austin, Texas.
The Hubble analysis also helped the astronomers to independently
verify the theory of how a white dwarf’s radius is determined by its
mass, an idea first proposed in 1935 by Indian American astronomer
Subrahmanyan Chandrasekhar. “Our measurement is a nice confirmation of
white-dwarf theory, and it even tells us the internal composition of a
white dwarf,” said team member Howard Bond of Pennsylvania State
University in University Park.
Sahu’s team identified Stein 2051 B and its background star after
combing through data of more than 5,000 stars in a catalog of nearby
stars that appear to move quickly across the sky. Stars with a higher
apparent motion across the sky have a greater chance of passing in front
of a distant background star, where the deflection of light can be
measured.
After identifying Stein 2051 B and mapping the background star field,
the researchers used Hubble’s Wide Field Camera 3 to observe the white
dwarf seven different times over a two-year period as it moved past the
selected background star.
The Hubble observations were challenging and time-consuming. The
research team had to analyze the white dwarf’s velocity and the
direction it was moving in order to predict when it would arrive at a
position to bend the starlight so the astronomers could observe the
phenomenon with Hubble.
The astronomers also had to measure the tiny amount of deflected
starlight. “Stein 2051 B appears 400 times brighter than the distant
background star,” said team member Jay Anderson of STScI, who led the
analysis to precisely measure the positions of stars in the Hubble
images. “So measuring the extremely small deflection is like trying to
see a firefly move next to a light bulb. The movement of the insect is
very small, and the glow of the light bulb makes it difficult to see the
insect moving.” In fact, the slight movement is about 1,000 times
smaller than the measurement made by Eddington in his 1919 experiment.
Stein 2051 B is named for its discoverer, Dutch Roman Catholic priest
and astronomer Johan Stein. It resides 17 light-years from Earth and is
estimated to be about 2.7 billion years old. The background star is
about 5,000 light-years away.
The researchers plan to use Hubble to conduct a similar microlensing
study with Proxima Centauri, our solar system’s closest stellar
neighbor.
The team’s result will appear in the journal Science [advance online paper ] on June 9.
The Hubble Space Telescope is a project of international cooperation
between NASA and ESA (European Space Agency). NASA’s Goddard Space
Flight Center in Greenbelt, Maryland, manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore conducts Hubble science
operations. STScI is operated for NASA by the Association of
Universities for Research in Astronomy, Inc., in Washington, D.C.
Related Links
This site is not responsible for content found on external links
- The science paper by K. Sahu et al.
- NASA's Hubble Portal
- American Astronomical Society
- The Science paper (advance online publication)
Contacts
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Kailash Sahu
Space Telescope Science Institute, Baltimore, Maryland
410-338-4930
ksahu@stsci.edu
Source: HubbleSite/News
