Gaia Mission Measures Metals In Milky Way Stars

Space.com

Milky Way stars that are high and low in metallicity have been mapped by  the ESA Gaia mission.   Credit: ESA/Gaia/DPAC

Transcript

00:00Gaia's main objective is to determine the positions, motions and distances of billions of stars.

00:20When the images of the stars from Gaia's telescopes move across the focal plane,

00:25their positions at a given time are determined by the light-sensitive astrometric CCDs.

00:36Subsequently, the light from the star passes through prisms, producing low-resolution spectra,

00:42helping us to determine, for example, the temperature of the stars.

00:46Finally, the diffraction grating and carefully crafted lenses in the radial-velocity spectrograph

00:52disperse the light into high-resolution spectra,

00:55allowing us to determine the speed of the stars along the line of sight and their chemical composition.

01:05This animation presents some of these stellar spectra,

01:08where the brightness of the stars is shown as a function of wavelength.

01:12The variations are due to the light absorption from atoms and molecules present in the stellar atmosphere.

01:18Most of the ordinary matter in the universe consists of the lightest elements,

01:24hydrogen or helium, created during the Big Bang.

01:27For all heavier elements, such as calcium and iron, astronomers use the word metals.

01:33Most of these metals were created by nuclear fusion in stars,

01:38and given back to the interstellar medium, for example, by stellar winds and supernova explosions.

01:43In this way, our Milky Way is enriched in metals over the course of time.

01:52Here we are using the metal abundances derived from the RVS to colour the stars.

01:57Blue represents a low metallicity, red a high one. Green lies in between.

02:04The stars shown here are those for which the chemical compositions could be determined with the RVS spectrograph.

02:09Older stars should contain only a small amount of metals,

02:14while stars born later should have a higher metallicity.

02:18We now travel thousands of light years towards the centre of our Milky Way,

02:22and observe stars with very different amounts of metals in their atmospheres.

02:27Now we fly out of the plane of our galaxy,

02:38and look down on the Milky Way from above.

02:44In order to see all of the stars of our sample,

02:48we enhance their brightness and move closer to them.

02:52The distribution of metals, shown here, results from the mixture of stars of different luminosities.

03:01Gaia can detect dwarf stars with very low luminosities only if they're very close to us.

03:10Therefore, we now select only the very luminous giant stars in our sample,

03:15which can be detected by Gaia even at a distance of several thousand light years.

03:20We see that the enrichment in metals decreases as we move from the galactic centre to the outer galactic regions.

03:27This informs us about the chemical composition of the gas from which these stars were formed

03:33over more than 12 billion years of galactic history.

03:36Therefore, and thanks to the high level of detail of these Gaia observations,

03:41we can infer the rate at which the stars were born,

03:44the arrival of gas from the intergalactic regions,

03:47and the migration of stars inside the disk.

03:51The next sample consists of very young stars,

03:54only a few hundred million years old,

03:57and therefore about four billion years younger than our Sun.

04:01They are located along curves that reveal the spiral arms of the Milky Way

04:06where these stars were formed.

04:08The Sun is in a region outside the spiral arms.

04:11We see again the decrease in the metal enrichment as we look further outwards in our galaxy.

04:16This is the largest sample of young stars for which we have a detailed chemical description,

04:21thanks to Gaia Data Release 3.

04:23Because there are fewer of the young stars, we can show more of them individually.

04:27This allows us to visualise the motion of the stars as measured by Gaia.

04:34This short sequence corresponds to five million years.

04:38We see that the stars move together,

04:41illustrating the stellar motions in the disk of our Milky Way.

04:44Let us now move to the plane of our Milky Way and see our galaxy edge on.

04:52First, we look again at our full sample of stars for which the chemical compositions could be determined by Gaia Data Release 3.

05:04In the following, we will split this sample into the same groups as before.

05:09Now we show the sample of giant stars edge on.

05:12These luminous stars allow us to determine the chemical profile of the Milky Way disk,

05:17including its older stellar populations, far from the galactic plane.

05:21As we move outwards from the galactic centre,

05:24the disk density and apparent thickness decreases, like the chemical enrichment.

05:29In addition, in the inner regions, the stars near the galactic plane are more enriched in metals than the older stars,

05:36at higher distances above and below the plane.

05:39This is the sample of young stars, shown from the side.

05:45The stars in the spiral arms are located in the so-called thin disk,

05:50which has gas and ongoing star formation and to which our sun belongs.

05:55This thin disk profile becomes thicker as we move outwards from the galactic centre.

06:00Again, we show how these stars will move during the next five million years.

06:08We can see the disk rotation with the stars approaching in our direction.

06:12Up to now, we have shown the overall global enrichment in chemical species in the atmospheres of the stars.

06:22However, we have also determined individual abundances of chemical elements.

06:28As an example, we colour-code here the amount of calcium in the young stars,

06:34an element which is, for instance, important for the stability of our bones.

06:47Finally, we show a group of stars that has no strong concentration towards the galactic plane.

06:53Almost all of them are very poor in metals, and therefore shown as blue in this video.

06:58The stars were identified by their peculiar motion and chemical composition.

07:03They are the remains of a dwarf galaxy, called Gaia Enceladus,

07:07that merged with our Milky Way about 8 to 11 billion years ago.

07:12These stars illustrate that the galaxy in which we live is an ever-changing system,

07:18formed thanks to the assembly of stars and gas of different origins.

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