The galactic great thing about star formation

I had never seen galaxy images like this before. Nobody had! These images highlight regions of star formation in nearby galaxies. There are still a number of unanswered questions about how star formation actually works. To answer these questions, we observe galaxies that are actively forming stars in giant gas clouds. Until recently, we did not have the necessary resolution to clearly image the individual gas clouds themselves. But images released by a project called PHANGS (Physics at High Angular Resolution in Near GalaxieS) in a collaboration between the European Southern Observatory Very Large Telescope and the Atacama Large Millimeter / Submillmeter Array (ALMA) have details never seen before of star formation clouds shown in other galaxies.

This image combines observations of the nearby galaxies NGC 1300, NGC 1087, NGC 3627 (top, left to right), NGC 4254, and NGC 4303 (bottom, left to right), captured with the Multi-Unit Spectroscopic Explorer (MUSE) ESO’s Very Large Telescope (VLT). Each individual image is a combination of observations made at different wavelengths of light to map star populations and warm gas. Picture and picture description PHANGS / ESO. Original picture

A cloud of stardust

Stars are formed from giant molecular clouds (GMCs), which are mainly composed of molecular hydrogen (H2). Gas in these clouds collapses under gravity and eventually becomes dense spheres. Due to the increase in density and pressure, the heat in these spheres enables nuclear fusion, in which hydrogen is fused into helium – a star is born! But what triggers the initial collapse of the gas? Does the star formation rate vary between different clouds in the same galaxy? How diverse are the clouds themselves? These are all chapters of star formation that we are not entirely sure about. Enter PHANGS.

The PHANGS researchers selected the target galaxies on the basis of a number of requirements. The galaxies had to be close enough that they could be imaged with the necessary resolution to see individual GMCs. All targets are therefore within 17 million parsecs of the Milky Way (about 55 million light years). The galaxies are also not tilted too much to allow a clear line of sight into the disks of the target galaxies. And perhaps most importantly, the target galaxies are actively forming stars. As “main sequence galaxies”, these galaxies form stars in their disks without the external gravitational interaction of a nearby galaxy or as a result of galaxy mergers, both of which can trigger intense phases of star formation, known as starbursts. Rather, these galaxies form stars through processes within the galaxy. Ninety such galaxies met the criteria and were selected for the survey.

These contrasting images show the increased resolution in carbon monoxide detection. The left side shows previous surveys of cold gas clouds in the galaxy NGC 3627 compared to the increased resolution of the “cloud scale” achieved by PHANGS-ALMA, which shows a much clearer picture of the GMC positions in the galaxy. c PHANGS-ALMA

Cold and dark

The discovery of star formation regions in the target galaxies is achieved through a combination of the discovery of cold gas and hot gases heated by newly formed stars. Cold GMCs that give birth to new stars are known as star nurseries. They can be tens to hundreds of light years in diameter, with mass equivalent to thousands of suns. However, the hydrogen that makes up these clouds is difficult to see. When hydrogen is exposed to energy, it glows and is easily detectable, while cold hydrogen hides in the darkness of space. However, GMCs also contain carbon monoxide (CO), which is easier to spot when cold than hydrogen. The ratio of CO to hydrogen in GMCs is understood as a constant and so the amount of CO molecule detected can tell us how much hydrogen is in a given cloud. It is this CO signal that ALMA is looking for.

This picture shows the distribution of cold (CO) vs. hot (H-alpha) gas spread across multiple galaxies (the colors in this diagram are not intuitive). The cold CO gas signatures are mapped by ALMA, while the scorching hot H-alpha is mapped by the VLT. The combined map shows where emerging stars are being born in the cold GMCs. c PHANGS-ALMA

As soon as hydrogen is excited by the energies of newly emerging and young stars, it releases a light known as Hydrogen Alpha. H-alpha is the brightest feature in the spectrum of glowing hydrogen and is the way we observe much of the universe. The combination of the hot and cold maps of these GMCs in other galaxies shows the environment in which stars form. An instrument called MUSE on the Very Large Telescope maps the glowing H-alpha where ALMA detects the cold CO emissions. The finest details resolved by ALMA in the target galaxies have a diameter of about 100 parsecs (about 326 light years). The researchers note that this is a “cloud-scale” resolution because the target GMCs are also about 100 parsecs in diameter. With this resolution, the clouds can be distinguished as individual structures separately from the structures of their other home galaxies.

ESO’s Very Large Telescope (VLT) shows the nearby galaxy NGC 4303, a spiral galaxy with a bar of stars and gas at its center, located about 55 million light years from Earth in the constellation of Virgo. The golden glow mostly corresponds to clouds of hot hydrogen, which indicate the presence of newly born stars, while the bluish regions in the background reveal the distribution of slightly older stars.
C. PHANGS / ESO – original image

A next generation card

While VLT takes pictures in optical light, ALMA sees distant galaxies in infrared and radio wavelengths. These wavelengths are useful for observing structures that would not be visible in optical wavelengths such as cold gas. But there is a downside. Optical wavelengths can typically provide finer resolution for imaging, creating a tradeoff between visibility and resolution. The impressive achievement of this initiative is that these new ALMA images achieved a resolution in infrared and radio that was close to optical resolutions. The images are further enhanced by merging the optical resolutions of the VLT as well as data from the Hubble Space Telescope with other images.

NGC 4254 ALMA (orange / red) data from cold GMC clouds applied to data from the Hubble Space Telescope,
Photo credits: ALMA (ESO / NAOJ / NRAO) / PHANGS, S. Dagnello (NRAO) Original image mosaic of the ALMA recognition of star children in combination with Hubble space telescope data. The pictures show the diversity of the GMC star formation clouds from the nearby universe. C. ALMA (ESO / NAOJ? NRAO) / PHANGS, S. Dangnell (NRAO)

The level of detail is overwhelming. These are not individual photos of each galaxy, but mosaics. For comparison: The Trottier Observatory I am working on was able to depict Andromeda, a much closer galaxy with 2.4 million light years, in a six-photo mosaic. Every galaxy that was mapped in the PHANGS-ALMA project, despite its distance of tens of millions of light years, consists of mosaics consisting of up to two hundred individual images. The process of mapping all 90 galaxies at this level of detail spanned a total of 6 years, resulting in a new atlas of star nurseries – the next generation of stars to be born in the universe.

How are stars formed? – Video by Fraser Cain

100,000 star kindergartens were mapped between the 90 target galaxies. The results show that the position in a galaxy can change the way stars are formed. Clouds in central regions of the galaxy are more massive, denser and more turbulent than those in the expanses of the galaxy disk. The rate at which the clouds form stars, and the resulting final dissolution of the cloud by these new stars blowing the gas away, all seem to vary depending on where the cloud is in its home galaxy.

Atacma Large Millimeter / Submillimeter Array (ALMA) with a dramatic meteor overhead.
C. ESO / C. Malin

A common view

The facilities required to capture these images are powerful and quite picturesque in themselves. ALMA is not a large telescope, but an arrangement of 66 dishes spread over the Chajnantor Plateau of the Atacama Desert in Chile. The signals collected by the array are effectively combined to create a huge bowl. The individual dishes can also be rearranged depending on the needs of the respective project. The Very Large Telescope is also located in the Atacama and consists of four telescopes, two with 8.2m mirrors and two smaller 1.8m mirrors. Like ALMA, the telescopes work together effectively creating a larger telescope.

Telescope “Yepun”, one of 4 of the VLT telescopes shown here, which fires a laser with adaptive optics into the sky, which compensates for the distortion of the atmosphere and creates a sharper image. C. ESO / Y. Beletsky

Although these are the most detailed images of their kind, the resolution achieved by PHANGS-ALMA is just on the threshold that is required to image individual GMCs in the target galaxies. But now that these star-forming regions have been mapped, future telescopes like the upcoming James Webb Space Telescope and the Extremely Large Telescope (the next one will be Ultra Large, I assume) will be able to locate these GMCs with sufficient resolution to visit again to look into the clouds themselves for even more insight into star formation.

Cosmic origins

We were once part of a giant molecular cloud, you and me. Everything your body includes, the computer you are reading this on and the planet we live on started out of a huge cloud of stardust. The future of telescopic space exploration is so tempting. James Webb will not only have the opportunity to study star formation in nearby galaxies, but also to image some of the first stars ever to be born in the universe. The main mirror of the ELT will have a diameter of 39 meters! Its surrounding dome is the size of a soccer field. We are on the brink of views of the universe we have never seen before and ultimately new understandings of our own origins in the cosmos.

Feature picture: Image of the galaxy NGC 3627 in the constellation LEO. The golden gas glow corresponds to clouds of ionized hydrogen, while the bluish regions reveal the distribution of slightly older stars. Photo credit: ESO / PHANGS

More to explore:

Galactic Fireworks: New ESO Images Reveal Breathtaking Features of Nearby Galaxies | ESO (With the original pictures)

Cosmic cartographers map the universe nearby, revealing the diversity of star-forming galaxies – National Radio Astronomy Observatory (

[2104.07739] PHANGS-ALMA: Arcsecond CO (2-1) imaging of nearby star-forming galaxies ( (Open Access original research paper)


A galaxy makes new stars faster than its black hole can starve for fuel – Universe Today

The universe in formation. Hubble sees 6 examples of the merging of galaxies – universe today


Atacama Large Millimeter / Submillimeter Array | ALMA (


A powerful new laser will improve adaptive optics – Universe Today

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