Milky Way stars that are high and low in metallicity have been mapped by the ESA Gaia mission.
Credit: ESA/Gaia/DPAC
Credit: ESA/Gaia/DPAC
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TechTranscript
00:00Gaia's main objective is to determine the positions, motions and distances of billions
00:19of stars.
00:20When the images of the stars from Gaia's telescopes move across the focal plane, their
00:25positions 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, allowing us to determine the speed of the
00:56stars along the line of sight and their chemical composition.
01:05This animation presents some of these stellar spectra where the brightness of the stars
01:10is shown as a function of wavelength.
01:12The variations are due to the light absorption from atoms and molecules present in the stellar
01:17atmosphere.
01:20Most of the ordinary matter in the universe consists of the lightest elements, hydrogen
01:25or helium, created during the Big Bang.
01:28For all heavier elements, such as calcium and iron, astronomers use the word metals.
01:34Most of these metals were created by nuclear fusion in stars and given back to the interstellar
01:39medium, 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
02:08with the RVS spectrograph.
02:11Older stars should contain only a small amount of metals, while stars born later should have
02:16a higher metallicity.
02:18We now travel thousands of light years towards the centre of our Milky Way and observe stars
02:24with very different amounts of metals in their atmospheres.
02:36Now we fly out of the plane of our galaxy and look down on the Milky Way from above.
02:46In order to see all of the stars of our sample, we enhance their brightness and move closer
02:51to them.
02:54The distribution of metals shown here results from the mixture of stars of different luminosities.
03:02Gaia can detect dwarf stars with very low luminosities only if they are very close to us.
03:10Therefore we now select only the very luminous giant stars in our sample, which can be detected
03:16by 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
03:25outer 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.
03:37Therefore, and thanks to the high level of detail of these Gaia observations, we can
03:41infer the rate at which the stars were born, the arrival of gas from the intergalactic
03:46regions and the migration of stars inside the disk.
03:51The next sample consists of very young stars, only a few hundred million years old, and
03:57therefore about 4 billion years younger than our Sun.
04:01They are located along curves that reveal the spiral arms of the Milky Way where these
04:07stars 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.
04:30This allows us to visualise the motion of the stars as measured by Gaia.
04:35This short sequence corresponds to 5 million years.
04:39We see that the stars move together, illustrating the stellar motions in the disk of our Milky
04:44Way.
04:48Let us now move to the plane of our Milky Way and see our galaxy edge-on.
04:56First we look again at our full sample of stars for which the chemical compositions
05:00could 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.
05:13These luminous stars allow us to determine the chemical profile of the Milky Way disk,
05:18including its older stellar populations, far from the galactic plane.
05:21As we move outwards from the galactic centre, the disk density and apparent thickness decreases
05:27like the chemical enrichment.
05:30In addition, in the inner regions, the stars near the galactic plane are more enriched
05:34in metals than the older stars, at higher distances above and below the plane.
05:41This 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, which has gas and
05:51ongoing star formation and to which our Sun belongs.
05:55This thin disk profile becomes thicker as we move outwards from the galactic centre.
06:04Again we show how these stars will move during the next 5 million years.
06:08We can see the disk rotation, with the stars approaching in our direction.
06:16Up to now we have shown the overall global enrichment in chemical species in the atmospheres
06:21of the stars.
06:23However, we have also determined individual abundances of chemical elements.
06:29As an example, we colour-code here the amount of calcium in the young stars, an element
06:34which 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
06:52plane.
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, that merged with our Milky
07:08Way about 8 to 11 billion years ago.
07:13These 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.
07:33Transcription by ESO. Translation by —