Image of the Week


the Galaxy in your preferred colours



Figure 1. All-sky map (Hammer projection) in the JWST NIRCam photometric system. The plot shows the median of the color F070W - F090W in each HEALPix (level=7), as measured from the Gaia XP spectra using synthetic photometry. Image credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO. Acknowledgements: R. Sordo, F. De Angeli, M. Riello.


The full 3rd Data Release of Gaia will provide to the community an impressive amount of ~220 million XP spectra, both internally and externally calibrated (De Angeli et al 2022, Montegriffo et al 2022). Spectra for all types of sources will be represented, as long as visible at the limiting magnitude: the database of spectra will be unprecedented in the combination of both precision in the flux calibration and variety. From the Gaia XP spectra, a source classification and astrophysical parameter determination will be presented too (Creevey et al 2022).

In the paper Gaia Collaboration, Montegriffo et al 2022, “Gaia Data Release 3: The Galaxy in your preferred colours. Synthetic photometry from Gaia low-resolution spectra”, published at the time of the Data Release 3, the potential of the Gaia low resolution spectra is exploited further, by doing what astronomers are used to do since the dawn of astrophysics: photometry.


Why do we need new photometry?

Even if the integrated Gaia photometry is exquisite and accurate at the milli-mag level, and has already allowed the community to explore the whole sky in an unprecedented way, it has some drawbacks. The Gaia G, GBP and GRP passbands are very large and their diagnostic capability is not tailored to specific scientific problems, as it is usually the case for photometric surveys.

In fact, astronomers looked at the sky for a century, devising photometric systems specifically designed for diagnostic purposes, or to characterize unknown sources. Photometry is quicker and allows us to go further in distance than spectroscopy, the latter providing a wealth of more information, but at a cost. With the Gaia spectra, we can overcome these limitations in the Gaia photometric system, and really go further, taking advantage of some key properties of the Gaia spectral database.

First, the spectra collected by Gaia are calibrated in flux units with a precision difficult to find in literature except for very limited samples. Second, those spectra are distributed in the whole sky, being Gaia an all-sky space-based survey. Third, there is no pre-selection on observed targets, so that all source types are represented, and their Gaia XP spectra are calibrated in the same way.

The externally calibrated XP spectra can be coupled to a set of (optical) passbands: the resulting synthetic photometry can reach the typical accuracy of 1-2% with a precision rivalling the best available photometry from literature in the same system. Using an existing set of reliable photometric standard stars as "second level calibrators" also the accuracy can be pushed to the millimag level, in some cases.


How precise is this photometry?

The key to precision and accuracy is in the excellent quality of the calibration of the spectra, which is one of the most difficult tasks in astronomy. The process is detailed in its complexity in De Angeli et al 2022 and Montegriffo et al 2022. Albeit minute systematic differences remain, as addressed in Montegriffo et al 2022, the agreement of the Gaia BP/RP spectra with other, well calibrated sets of observed spectra from literature is very high. The Data Processing and Analysis Consortium (DPAC) offers to the community a tool, GaiaXPy, that, among other functionalities, allows every user to perform synthetic photometry on the large set of Gaia XP spectra published in Gaia DR3, by plugging in any existing (or desired!) photometric system covering the optical wavelength range.

The paper Gaia Collaboration, Montegriffo et al 2022 explains the theoretical foundation of the method that allows GaiaXPy to easily, quickly and reliably compute synthetic photometry from Gaia BP/RP spectra: an apparently simple procedure that is indeed quite tricky to perform correctly! The paper presents also a thorough assessment of the quality of these measures for a set of well-known photometric systems, showing detailed comparisons with the best available samples of their reference photometric measures.

Finally, a catalogue of these measures (Gaia Synthetic Photometric Catalogue, GSPC) in a set of well-known photometric systems is published as part of the Gaia DR3, for the ~220 million sources having their Gaia XP mean spectra in Gaia DR3. This catalogue can be queried through the Gaia archive. The GSPC can be used to provide a reference for any optical photometric measure, and for the calibration/validation of photometric surveys.

From the GSPC, the detailed colour-magnitude diagram shown in Figure 2 was constructed, by combining measures in the Johnson-Kron-Cousins B and I passbands for a sample of nearly 4 million stars situated in the Galactic Caps (|b| > 50deg). Well-known features, associated with specific evolutionary states of stars, can be recognized thanks to the high accuracy of the photometric measures.


Figure 2. Colour-magnitude diagram generated by combining the Johnson-Kron-Cousins (B-I) colour and the absolute magnitude in the I band, as measured from the Gaia parallax and the Gaia XP spectra using synthetic photometry. The main loci of the stellar evolutions are marked (MS: Main Sequence; RGB: Red Giant Branch; TRGB: Tip of the RGB; AGB: Asymptotic Giant Branch HB: Horizontal Branch; EHB: Extreme HB; WD: White Dwarfs). Image credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO. Acknowledgements: M. Bellazzini.


The sky in your preferred colours

Synthetic photometry from XP spectra allows us to generate sky maps using our preferred colour computed from optical passbands. We divide the sky in (HEALPix, level 7) pixels, and we take for each pixel the median colour of all associated stars within. Using GSPC measures, in Figure 1 we are able to take a glimpse on the JWST world. Even if the first real measurements are yet to come, by implementing in GaiaXPy its optical passbands F090W and F070W and simulating its forthcoming measures we can already see our sky through the eyes of the JWST NIRCam instrument.​​​​



  • De Angeli et al 2022, “Gaia Data Release 3: Processing and validation of BP/RP low-resolution spectral data”
  • Montegriffo et al 2022 “Gaia Data Release 3: External calibration of BP/RP low-resolution spectroscopic data”
  • Gaia collaboration, Montegriffo et al 2022 “Gaia Data Release 3: The Galaxy in your preferred colours: synthetic photometry from Gaia low-resolution spectra”
  • Creevey et al 2022, “Gaia Data Release 3: Astrophysical parameters inference system (Apsis) I - methods and content overview”



The data used to prepare this material are presented in the paper Gaia collaboration, Montegriffo et al 2022 “Gaia Data Release 3: The Galaxy in your preferred colours: synthetic photometry from Gaia low-resolution spectra”


Credits: ESA/Gaia/DPAC, R. Sordo, M. Bellazzini, F. De Angeli, M. Riello, A. Vallenari, T. Roegiers

[Published: 23/05/2022]

Image of the Week Archive


28/05: Did Gaia find its first neutron star?

26/04: A textbook solar eruption

22/04: Gaia's contribution to discovering distant worlds

16/04: Gaia spots Milky Way's most massive black hole of stellar origin

02/04: The Gaia Cataclysmic Variable hook


19/12: 10 Science topics to celebrate Gaia's 10 years in space

31/10: Gaia observes cosmic clock inside a heavenly jewel

10/10: Gaia Focused Product Release stories

27/09: Does the Milky Way contain less dark matter than previously thought?

22/09: Mass-luminosity relation from Gaia's binary stars

13/09: Gaia DPAC CU8 seminars

13/06: Gaia's multi-dimensional Milky Way

18/05: Mapping the Milky Way

15/05: Goonhilly station steps in to save Gaia science data

25/04: The Gaia ESA Archive

05/04: Dual quasar found to be hosted by an ongoing galaxy merger at redshift 2.17

21/03: GaiaVari: a citizen science project to help Gaia variability classificaton

09/02: Missing mass in Albireo Ac: massive star or black hole?

31/01: Gaia reaches to the clouds – 3D kinematics of the LMC

25/01: Meet your neighbours: CNS5 - the fifth catalogue of nearby stars

18/01: A single-object visualisation tool for Gaia objects


25/11: 100 months of Gaia data

23/11: The astonishment

09/11: Gamma-Ray Burst detection from Lagrange 2 point by Gaia

04/11: Gaia's first black hole discovery: Gaia BH1

26/10: Are Newton and Einstein in error after all?

21/10: Gaia ESA Archive goes live with third data release

06/10: Mapping the interstellar medium using the Gaia RVS spectra

26/09: Gaia on the hunt for dual quasars and gravitational lenses

23/09: Gaia's observation of relativistic deflection of light close to Jupiter

13/06: Gaia Data Release 3

10/06: MK classification of stars from BP/RP spectrophotometry across the Hertzsprung-Russell diagram

09/06: BP/RP low-resolution spectroscopy across the Hertzsprung-Russell diagram

27/05: Cepheids and their radial velocity curves

23/05: The Galaxy in your preferred colours

19/05: GaiaXPy 1.0.0 released, a tool for Gaia's BP/RP spectra users

11/05: Systemic proper motions of 73 galaxies in the Local group

28/03: Gaia query statistics

16/03: Gaia's first photo shooting of the James Webb Space Telescope

08/03: Gaia's women in science - coordination unit 8

25/02: Not only distances: what Gaia DR3 RR Lyrae stars will tell us about our Galaxy and beyond

11/02: Gaia's women in science

31/01: Astrometric orbit of the exoplanet-host star HD81040

12/01: The Local Bubble - source of our nearby stars

05/01: A Milky-Way relic of the formation of the Universe


23/12: Signal-to-Noise ratio for Gaia DR3 BP/RP mean spectra

22/12: The 7 October 2021 stellar occultation by the Neptunian system

01/12: Observation of a long-predicted new type of binary star

24/09: Astrometric microlensing effect in the Gaia16aye event

22/09: the power of the third dimension - the discovery of a gigantic cavity in space

16/09: An alternative Gaia sky chart

25/08: Gaia Photometric Science Alerts and Gravitational Wave Triggers

09/07: How Gaia unveils what stars are made of

23/06: Interviews with CU3

27/04: HIP 70674 Orbital solution resulting from Gaia DR3 processing

30/03: First transiting exoplanet by Gaia

26/03: Apophis' Yarkovsky acceleration improved through stellar occultation

26/02: Matching observations to sources for Gaia DR4


22/12: QSO emission lines in low-resolution BP/RP spectra

03/12: Gaia Early Data Release 3

29/10: Gaia EDR3 passbands

15/10: Star clusters are only the tip of the iceberg

04/09: Discovery of a year long superoutburst in a white dwarf binary

12/08: First calibrated XP spectra

22/07: Gaia and the size of the Solar System

16/07: Testing CDM and geometry-driven Milky Way rotation Curve Models

30/06: Gaia's impact on Solar system science

14/05: Machine-learning techniques reveal hundreds of open clusters in Gaia data

20/03: The chemical trace of Galactic stellar populations as seen by Gaia

09/01: Discovery of a new star cluster: Price-Whelan1

08/01: Largest ever seen gaseous structure in our Galaxy


20/12: The lost stars of the Hyades

06/12: Do we see a dark-matter like effect in globular clusters?

12/11: Hypervelocity star ejected from a supermassive black hole

17/09: Instrument Development Award

08/08: 30th anniversary of Hipparcos

17/07: Whitehead Eclipse Avoidance Manoeuvre

28/06: Following up on Gaia Solar System Objects

19/06: News from the Gaia Archive

29/05: Spectroscopic variability of emission lines stars with Gaia

24/05: Evidence of new magnetic transitions in late-type stars

03/05: Atmospheric dynamics of AGB stars revealed by Gaia

25/04: Geographic contributions to DPAC

22/04: omega Centauri's lost stars

18/04: 53rd ESLAB symposium "the Gaia universe"

18/02: A river of stars

21/12: Sonification of Gaia data
18/12: Gaia captures a rare FU Ori outburst
12/12: Changes in the DPAC Executive
26/11:New Very Low Mass dwarfs in Gaia data
19/11: Hypervelocity White Dwarfs in Gaia data
15/11: Hunting evolved carbon stars with Gaia RP spectra
13/11: Gaia catches the movement of the tiny galaxies surrounding the Milky Way
06/11: Secrets of the "wild duck" cluster revealed
12/10: 25 years since the initial GAIA proposal
09/10: 3rd Gaia DPAC Consortium Meeting
30/09: A new panoramic sky map of the Milky Way's Stellar Streams
25/09: Plausible home stars for interstellar object 'Oumuamua
11/09: Impressions from the IAU General Assembly
30/06: Asteroids in Gaia Data
14/06: Mapping and visualising Gaia DR2

25/04: In-depth stories on Gaia DR2

14/04: Gaia tops one trillion observations
16/03: Gaia DR2 Passbands
27/02: Triton observation campaign
11/02: Gaia Women In Science
29/01: Following-up on Gaia
19/12: 4th launch anniversary
24/11: Gaia-GOSA service
27/10: German Gaia stamp in the making
19/10: Hertzsprung-russell diagram using Gaia DR1
05/10: Updated prediction to the Triton occultation campaign
04/10: 1:1 Gaia model arrives at ESAC
31/08: Close stellar encounters from the first Gaia data release
16/08: Preliminary view of the Gaia sky in colour
07/07: Chariklo stellar occultation follow-up
24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
27/08: Quest for the Sun's siblings
Please note: Entries from the period 2003-2010 are available in this PDF document.