The star Alpha Draconis (circled), also known as Thuban, has long been known to be a binary system. Now data from NASA's TESS show its two stars undergo mutual eclipses. Credit: NASA/MIT/TESS
Astronomers using data from NASA’s Transiting Exoplanet Survey Satellite (TESS)
have shown that Alpha Draconis, a well-studied star visible to the
naked eye, and its fainter companion star regularly eclipse each
other. While astronomers previously knew this was a binary system, the
mutual eclipses came as a complete surprise.
“The first question that comes to mind is ‘how did we miss this?’”
said Angela Kochoska, a postdoctoral researcher at Villanova University
in Pennsylvania who presented the findings at the 235th meeting of the
American Astronomical Society in Honolulu on Jan. 6. “The eclipses are
brief, lasting only six hours, so ground-based observations can easily
miss them. And because the star is so bright, it would have quickly
saturated detectors on NASA’s Kepler observatory, which would also mask
the eclipses.”
This animation illustrates a preliminary model of
the Thuban system, now known to be an eclipsing binary thanks to data
from NASA’s Transiting Exoplanet Survey Satellite (TESS). The stars
orbit every 51.4 days at an average distance slightly greater than
Mercury’s distance from the Sun. We view the system about three degrees
above the stars’ orbital plane, so they undergo mutual eclipses, but
neither is ever completely covered up by its partner. The primary star
is 4.3 times bigger than the Sun and has a surface temperature around
17,500 degrees Fahrenheit (9,700 C), making it 70% hotter than our Sun.
Its companion, which is five times fainter, is most likely half the
primary’s size and 40% hotter than the Sun. Thuban, also called Alpha
Draconis, is located about 270 light-years away in the northern
constellation Draco. Credits: NASA’s Goddard Space Flight Center/Chris Smith (USRA). Download HD video from NASA Goddard's Scientific Visualization Studio
The system ranks among the brightest-known eclipsing binaries where
the two stars are widely separated, or detached, and only interact
gravitationally. Such systems are important because astronomers can
measure the masses and sizes of both stars with unrivaled accuracy.
Alpha Draconis, also known as Thuban, lies about 270 light-years away in the northern constellation Draco. Despite its “alpha” designation, it shines as Draco’s fourth-brightest star. Thuban’s fame arises from a historical role it played some 4,700 years ago, back when the earliest pyramids were being built in Egypt.
At that time, it appeared as the North Star, the one closest to the
northern pole of Earth’s spin axis, the point around which all of the
other stars appear to turn in their nightly motion. Today, this role is
played by Polaris, a brighter star in the constellation Ursa Minor. The
change happened because Earth’s spin axis performs a cyclic 26,000-year
wobble, called precession, that slowly alters the sky position of the
rotational pole.
TESS monitors large swaths of the sky, called sectors, for 27 days at
a time. This long stare allows the satellite to track changes in
stellar brightness. While NASA’s newest planet hunter mainly seeks
dimmings caused by planets crossing in front of their stars, TESS data
can be used to study many other phenomena as well.
A 2004 report suggested that Thuban displayed small brightness
changes that cycled over about an hour, suggesting the possibility that
the system’s brightest star was pulsating.
To check this, Timothy Bedding, Daniel Hey, and Simon Murphy at the
University of Sydney, Australia, and Aarhus University, Denmark, turned
to TESS measurements. In October, they published a paper that described
the discovery of eclipses by both stars and ruling out the existence of pulsations over periods less than eight hours.
Now Kochoska is working with Hey to understand the system in greater detail.
“I've been collaborating with Daniel to model the eclipses and
advising on how to bring together more data to better constrain our
model.” Kochoska explained. “The two of us took different approaches to
modeling the system, and we hope our efforts will result in its full
characterization.”
As known from earlier studies, the stars orbit every 51.4 days at an
average distance of about 38 million miles (61 million kilometers),
slightly more than Mercury’s distance from the Sun. The current
preliminary model shows that we view the system about three degrees
above the stars’ orbital plane, which means neither star completely
covers the other during the eclipses. The primary star is 4.3 times
bigger than the Sun and has a surface temperature around 17,500 degrees
Fahrenheit (9,700 C), making it 70% hotter than our Sun. Its companion,
which is five times fainter, is most likely half the primary’s size and
40% hotter than the Sun.
Kochoska says she is planning ground-based follow-up observations and anticipating additional eclipses in future TESS sectors.
“Discovering eclipses in a well-known, bright, historically important
star highlights how TESS impacts the broader astronomical community,”
said Padi Boyd, the TESS project scientist at NASA’s Goddard Space
Flight Center in Greenbelt, Maryland. “In this case, the high precision,
uninterrupted TESS data can be used to help constrain fundamental
stellar parameters at a level we’ve never before achieved.”
TESS is a NASA Astrophysics Explorer mission led and operated by MIT
in Cambridge, Massachusetts, and managed by NASA's Goddard Space Flight
Center. Additional partners include Northrop Grumman, based in Falls
Church, Virginia; NASA’s Ames Research Center in California’s Silicon
Valley; the Harvard-Smithsonian Center for Astrophysics in Cambridge,
Massachusetts; MIT’s Lincoln Laboratory; and the Space Telescope Science
Institute in Baltimore. More than a dozen universities, research
institutes and observatories worldwide are participants in the mission.
By Francis Reddy
NASA’s Goddard Space Flight Center, Greenbelt, Md.
Media contact
Claire Andreoli
NASA’s Goddard Space Flight Center, Greenbelt, Md.
(301) 286-1940
Editor: Francis Reddy
Source: NASA/TESS
