Pinning down the Milky Way's spin

Sep 19, 2008
Artist's impression of the local neighbourhood of the Sun and its setting within our galaxy, the Milky Way (see insert above). The figure shows the positions of some bright stars (in white) in the sky as well as the eight Cepheids used in the investigation (in blue). After the rotation of the Milky Way had been accounted for (red arrow), it seemed that the Cepheids were all "falling" towards the Sun (blue arrows; these are not to scale: in reality the blue velocities are typically a factor one hundred smaller than the velocity around the Milky Way). New, very precise measurements with the HARPS instrument have shown that this apparent "fall" is due to effects within the Cepheids themselves and is not related to the way the Milky Way rotates. The motion indicated by the blue arrows is thus an illusion. The scale of the image is given in light-years (ly). Credit: ESO

New, very precise measurements have shown that the rotation of the Milky Way is simpler than previously thought. A remarkable result from the most successful ESO instrument HARPS, shows that a much debated, apparent 'fall' of neighbourhood Cepheid stars towards our Sun stems from an intrinsic property of the Cepheids themselves.

The result, obtained by a group of astrophysicists led by Nicolas Nardetto, will soon appear in the journal Astronomy & Astrophysics.

Since Henrietta Leavitt's discovery of their unique properties in 1912, the class of bright, pulsating stars known as Cepheids has been used as a distance indicator. Combined with velocity measurements, the properties of Cepheids are also an extremely valuable tool in investigations of how our galaxy, the Milky Way, rotates.

"The motion of Milky Way Cepheids is confusing and has led to disagreement among researchers," says Nardetto. "If the rotation of the Galaxy is taken into account, the Cepheids appear to 'fall' towards the Sun with a mean velocity of about 2 km/s."

A debate has raged for decades as to whether this phenomenon was truly related to the actual motion of the Cepheids and, consequently, to a complicated rotating pattern of our galaxy, or if it was the result of effects within the atmospheres of the Cepheids.

Nardetto and his colleagues observed eight Cepheids with the high precision HARPS spectrograph, attached to the 3.6-m ESO telescope at La Silla, 2400 m up in the mountains of the Chilean Atacama Desert. HARPS, or the High Accuracy Radial Velocity Planetary Searcher, is best known as a very successful planet hunter, but it can also be used to resolve other complicated cases, where its ability to determine radial velocities - the speed with which something is moving towards or away from us - with phenomenally high accuracy is invaluable. "Our observations show that this apparent motion towards us almost certainly stems from an intrinsic property of Cepheids," says Nardetto.

The astronomers found that the deviations in the measured velocity of Cepheids were linked to the chemical elements in the atmospheres of the Cepheids considered. "This result, if generalised to all Cepheids, implies that the rotation of the Milky Way is simpler than previously thought, and is certainly symmetrical about an axis," concludes Nardetto.

Source: ESO

Explore further: What's the brightest star in the sky, past and future?

add to favorites email to friend print save as pdf

Related Stories

VISTA stares right through the Milky Way

Feb 04, 2015

A new image taken with ESO's VISTA survey telescope reveals the Trifid Nebula in a new light. By observing in infrared light, astronomers can see right through the central parts of the Milky Way and spot ...

Hubble views the star that changed the universe

May 23, 2011

(PhysOrg.com) -- Though the universe is filled with billions upon billions of stars, the discovery of a single variable star in 1923 altered the course of modern astronomy. And, at least one famous astronomer ...

Hubble rules out one alternative to dark energy

Mar 14, 2011

(PhysOrg.com) -- Astronomers using NASA's Hubble Space Telescope have ruled out an alternate theory on the nature of dark energy after recalculating the expansion rate of the universe to unprecedented accuracy.

Recommended for you

Could the Milky Way become a quasar?

Feb 27, 2015

A quasar is what you get when a supermassive black hole is actively feeding on material at the core of a galaxy. The region around the black hole gets really hot and blasts out radiation that we can see billions ...

Galactic dinosaurs not extinct

Feb 27, 2015

One of the biggest mysteries in galaxy evolution is the fate of the compact massive galaxies that roamed the early Universe.

Stars found forming at Milky Way's outer edge

Feb 27, 2015

Brazilian astronomers said Friday they had found two star clusters forming in a remote part of our Milky Way galaxy where such a thing was previously thought impossible.

New insight found in black hole collisions

Feb 26, 2015

New research by an astrophysicist at The University of Texas at Dallas provides revelations about the most energetic event in the universe—the merging of two spinning, orbiting black holes into a much larger ...

User comments : 5

Adjust slider to filter visible comments by rank

Display comments: newest first

nano999
5 / 5 (4) Sep 19, 2008
here's the real image
http://www.eso.or...-08.html
aphelion
not rated yet Sep 19, 2008
thx nano999
gopher65
not rated yet Sep 19, 2008
It's a small thing, but none the less good to know. I'm also glad to note that the HARPS is turning up interesting information in fields other than what it was created for, rather than being limited to extra-solar planetary research (which is cool enough in and of itself, but still. It adds value).
deatopmg
1 / 5 (1) Sep 20, 2008
it's nice to have a confirmation that we really aren't the center of the universe.
lengould100
not rated yet Nov 21, 2008
Good. That was close, imagine the fuss if we had to organize expeditions to save the solar sysem from attacking Cephids. Makes re-directing colliding asteroids look like childs play.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.