Sky & Telescope - December 2018 - 32

Hypervelocity Stars

"We found that
the star was much
Total
faster than we thought
velocity
before," he says. The
Star
extreme Doppler shift
Tangential
had clocked only the
velocity
star's speed along our
Proper
line of sight. But Geier's
motion
team examined the
Earth
star's position on old
p 3D VELOCITY Although measurphotographic plates
ing the Doppler shift of spectral lines
to measure its proper
provides astronomers with an object's
motion, the apparent
radial velocity, it's really just a lower
movement across our
limit: Observers don't know its total
line of sight. Newly
3D velocity until they also measure its
proper motion and distance.
released data from the
Gaia mission give an
updated speed: The star darts through space at about 1,060
km/s. Furthermore, the trajectory shows that the star has
never been anywhere near the galactic center. So the Milky
Way's big black hole didn't eject the star.
The astronomers also discovered that US 708 spins fast.
Geier thinks the rapid spin arose because the star had been
tidally locked - it had circled a white dwarf every 10 minutes
and spun that fast itself, for the same reason that the Moon
spins as fast as it revolves. The star had once been a red giant,
but the white dwarf tore off the giant's hydrogen atmosphere,
leaving only the compact helium core we see today. When the
white dwarf exploded, US 708 ﬂew away.
Radial
velocity

And Then There Were Two
Meanwhile, other astronomers were tracking a strange star in
Ursa Minor named LP 40-365. "This one stood out as being
extremely fast," says Stéphane Vennes, then at the Academy
of Sciences of the Czech Republic. "In fact, a quick backof-the-envelope calculation showed that it was actually not
even bound to the Milky Way." Yet this star had also never
ventured near the galaxy's central black hole.

Unlike US 708, which an exploding white dwarf had
ﬂung away, LP 40-365 seems to be an actual white dwarf
that partially blew up. "It survived a supernova explosion,"
says Vennes. His team found chemical elements on the star
that its own explosion had created. As with a normal Type
Ia explosion, the white dwarf's partner dumped gas onto it,
but the resulting explosion ﬁ zzled, sparking a not-so-super
supernova, one that failed to completely destroy the white
dwarf. Now the wounded white dwarf sails through space on
its own, minus the partner that set off the ﬁreworks.
Another team, led by Roberto Raddi (Friedrich-Alexander
University, Germany), strengthened this conclusion by discovering that the star had manganese, an element that originates chieﬂy in Type Ia supernovae. Moreover, manganese
forms most readily when the exploding white dwarf's partner
is not a white dwarf itself.
The energy from the explosion puffed up the white dwarf,
making it larger than normal. In April, when the Gaia team
published its newest data release, Raddi's team found that the
star is about 2,060 light-years from Earth and moving at 850
km/s, well above the Milky Way's escape velocity. The distance yields the star's luminosity and diameter, both of which
are about a ﬁfth of the Sun's. This means the star is 15 times
larger than a typical white dwarf.

Life in the Fast Lane
Within hours of receiving the new Gaia data, Ken Shen
(University of California, Berkeley) and colleagues spotted
three more high-speed stars. All three are puffed-up white
dwarfs with strange chemical abundances: Unlike most white
dwarfs, their spectra show no hydrogen but instead lots of
carbon, oxygen, magnesium, and calcium.
"This is a super interesting discovery," says Geier, who was
not part of the study. "They are not only very weird; they are
also very similar to each other."
These stars are hurtling through space at incredible
speeds. The most extreme are D6-1 in Apus and D6-3 in
Draco. "These stars are probably the two fastest unbound

Swift and Small: The New Hypervelocity Stars
Constellation

US 708

Ursa Major

LP 40-365

Distance
(light-years)

Temperature
(kelvin)

Apparent
Magnitude*

Absolute
Magnitude*

(km/s)

28,000

47,000

18.7

+4.0

1,060

2.4 million

Ursa Minor

2,060

9000

15.6

+6.5

850

1.9 million

D6-1

Apus

6,900

8000

17.4

+5.5

2,300

5.1 million

D6-2

Pegasus

3,100

8000

17.0

+6.9

1,300

2.9 million

D6-3

Draco

7,600

8000

18.3

+6.3

2,400

5.4 million

*As measured by Gaia
**Relative to the Milky Way's rest frame. The Sun's velocity is about 250 km/s or 560,000 mph.

Velocity**
(mph)

DECE MBE R 2 018 * SK Y & TELESCOPE

LE AH TISCIONE / S&T

Designation

Sky & Telescope - December 2018

Table of Contents for the Digital Edition of Sky & Telescope - December 2018