How to take a Stellar Selfie

Our Home

Our Solar System is found in the Milky Way galaxy; a galaxy 100,000 light-years in diameter, with over 200 billion stars, of which the closest one to us, the sun, is 30,000 light-years from the galactic centre and tipped 63º off of the galactic plane (being like the Solar System’s ecliptic, which is the plane in which most objects orbit the Sun) [1].

It’s an amazing part of the universe, with a lovely spiral shape and a calm galactic centre (the central part of the galaxy, home to the supermassive black hole: Sagittarius A*).

But wait a second. How do we know all this, since we’re inside it. It’s rather like pretending to take a picture of Earth from the edge of the Solar System, and getting all the small details correct, whilst sat at home on Earth, but harder.

Milky Way over Black Rock Desert, Nevada, USA. Credit: Steve Jurvetson, CC BY 2.0

Before Satellites

Before the age of satellites, if we wanted to know what the place we live in looks like, it involved taking detailed measurements of distance and position, then creating a map of the galaxy.

The Herschels (William and Caroline) took on this task in the late 18th century, using a method called “star gauging”, reasoning that, if they saw many stars in one direction, it was the edge of the galaxy, and conversely, fewer stars in another is a closer patch of galaxy.

This did create a rather irregular shaped map, being because they thought that they could see to the edge of the galaxy, when they couldn’t. This is due to the interstellar dust clouds that hid some stars from their view [2] (being what also, incidentally, prevented us from finding Sagittarius A* for years). Also, they thought that stars were uniformly distributed and every star is as luminous as our Sun (both of which we now know aren’t correct), further making the observations invalid [3].

However flawed this method was, it did pave the way for others to take their place in the future, to then make more accurate models of the Milky Way.

Shape of Milky Way deduced from observations. Credit: Caroline Herschel/William Herschel


NASA’s WISE (Wide-field Infrared Survey Explorer), having had its 10th birthday in December 2019 [4], overcame problems experienced by the Herschels by using infrared light instead of visible, being able to discover some 400 or more stellar nurseries (where stars are formed from dust) once covered in a cloak of dust, but now revealed [5].

It can then map not just the Milky Way (seen in a characteristic belt across the sky) but the entire sky, seeing other galaxies in a new light (literally), showing the sky in much greater detail than ever before [4], creating data to then get used in the making of a map of our own galaxy.

Most importantly, it has led to finding that arms other than the 4 main ones are more important that previously thought, as well as finding better information as to what they look like, with the ability to find where gas and stars are very densely packed, which would otherwise be hidden by the dust surrounding them [5] (being what stopped the Herschels from mapping the Milky Way so well).

WISE data used to trace shape of spiral arms. Credit: NASA/JPL-Caltech


ESA’s Gaia spacecraft, 4 years younger than WISE, is different to WISE, in that it’s not only showing the structure of our galaxy, but how it formed and its future, using models from the public data to make many discoveries.

For instance, they found that the majority of stars in the Milky Way are orbiting clockwise, whilst there’s a large group of stars going anticlockwise, suggesting they weren’t born in this galaxy. They’ve also found that stars from the same stellar nursery moving in clusters [6].

As well as these amazing discoveries, Gaia data has helped in the finding, published on 25th May 2020, that the Sagittarius Dwarf Galaxy that began colliding with the Milky Way 5.7 billion years ago, has collided 3 times over the last 6 billion years, triggering star formation episodes, of which the Sun’s formation 4.7 billion years ago coincides, so the Sagittarius Dwarf Galaxy may be the reason we’re here today [7]. As well as this though, it was also found to be a reason for the Milky Way’s structure being not flat but warped (one part is curved upward and the other side is curved downward) [8].

And then, 2 days ago on 3rd December 2020, another paper used Gaia data to make a further groundbreaking observation, which used the mapped paths of stars in our Milky Way (specifically looking at how the anticentre, or edge of the galaxy, is growing and changing) to then get a prediction from a computer model of the next 400 thousand years of motion of stars in our galaxy, taking into account things like the Sagittarius Dwarf Galaxy, still being cannibalised by the Milky Way.

This, along with all the other numerous papers from Gaia data, show just how much of a “treasure trove for astronomers” (DE BRUIJNE Jos, 2020) this is [9], and will be for years to come, where we will hopefully have the ability to take even better stellar selfies.

How 40,000 stars in 100 parsecs will move in next 400 thousand years.

Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO

By George Abraham, ADAS member.

#MilkyWay #WISE #Gaia #Satellite #Herschel

Click here for the previous news article

Click here for the next news article

Click here for a list of virtual reality resources that take advantage of ESA’s Gaia data

Click here to look at ESA Sky using NASA’s WISE data

Click here to access optical colour ESA Sky, which has Gaia’s new data available.

Simply click on the 3rd icon along (after having selected “Science Mode”), which looks like an ellipse, and click on the “Gaia eDR3 (Optical)” square, before clicking on any green square on the map, when zoomed in to maximum, and clicking “Load Data”, showing all the observations Gaia has made, then showing you how much data Gaia collects, and so its potential for years to come.

N.B. As long as the name includes “Gaia”, it’s the correct data to look at (since the name may change over time, as more data is released).


  1. "Milky Way Galaxy: Facts About Our Galactic Home". Archived from the original on 5th December 2020.

  2. "Herschel and the Milky Way". Online Star Register. Archived from the original on 5th December 2020.

  3. "Mapping the Milky Way: William Herschel's Star Gages". Berry College. Archived from the original on 5th December 2020.

  4. "Celebrating 10 Years of the WISE Spacecraft". NASA. Archived from the original on 5th December 2020.

  5. "Charting the Milky Way from the Inside Out". NASA. Archived from the original on 5th December 2020.

  6. "Gaia Astronomical Revolution". ESA. Archived from the original on 5th December 2020.

  7. "Galactic Crash may have Triggered Solar System Formation". ESA. Archived from the original on 5th December 2020.

  8. "Milky Way's Warp caused by Galactic Collision, Gaia Suggests". ESA. Archived from the original on 5th December 2020.

  9. "Gaia's New Data takes us to the Milky Way's Anticentre and Beyond". ESA. Archived from the original on 5th December 2020.

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