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Starts With A Bang

Star clusters give birth like dogs, not humans, ALMA shows

Here in our Solar System, we only have one star: a singlet. For many systems, including the highest-mass ones, that’s anything but the norm.
A map of a cluster of stars illustrating star birth.
This false-color look inside the star-forming region G333.23–0.06 shows ALMA data of multiple systems of high-mass protostars. Within these clumps of matter, ALMA has found multi-star systems, with singlet stars being a relative rarity.
Credit: S. Li, MPIA / J. Neidel, MPIA Graphics Department; Data: ALMA Observatory
Key Takeaways
  • For a long time, we thought most of the stars and stellar systems in the sky were similar to our own: with a single, central star and only much lower-mass objects, like planets, orbiting around them.
  • Today we know that’s not true, as practically half of all known stars can be found as parts of multi-star systems: doublets, triplets, quadruplets and even richer configurations.
  • For the first time, thanks to ALMA, we’re able to probe how the most massive stars form, and singlets are rare, far outnumbered by multi-star systems. The most massive stars form like puppies, not baby humans.

Humanity once thought our Solar System was typical.

how many planets
Here in our own Solar System, a single star anchors the system, where inner, rocky planets, an intermediate-distance asteroid belt, and then more distant gas giant planets eventually give way to the Kuiper belt and Oort cloud. This configuration is not universal among stellar and planetary systems, which means our Solar System possesses many properties that are not necessarily common among exoplanetary systems.
Credit: NASA/Dana Berry

The other stars, presumably, were Sun-like objects, but very far away.

brightness distance
The brightness distance relationship, and how the flux from a light source falls off as one over the distance squared. The earliest estimates for the distances to the stars assumed they were intrinsically as bright as the Sun, and that their faint appearance was solely caused by their great distance from us.
Credit: E. Siegel/Beyond the Galaxy

We soon learned that stars and stellar systems varied tremendously.

morgan keenan spectral classification
The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin. In terms of size, the smallest M-class stars are still about 12% the diameter of the Sun, but the largest main sequence stars can be dozens of times the Sun’s size, with evolved red supergiants (not shown) reaching hundreds or even 1000+ times the size of the Sun. A star’s (main sequence) lifetime, color, temperature, and luminosity are all primarily determined by a single property: mass.
Credit: LucasVB/Wikimedia Commons; Annotations: E. Siegel

Individual stars come in many different masses, temperatures, and colors.

binary system
Binary systems typically have unequal masses, unequal brightnesses, and orbit a barycenter that lies outside of both stars. Only if the alignment with respect to us is sufficiently edge-on, at right, will it appear as an eclipsing binary. Wide binaries, with separations of thousands of astronomical units (AUs), are exceptionally difficult to characterize. Approximately 35% of all stars are found in binary systems, with half in singlet systems and the remainder in trinary or even richer multi-star systems.
Credit: Zhatt and Stanlekub/Wikimedia Commons

While our Solar System has just one star, half of all stellar systems have multiples.

Castor, the 6-star birth system infographic.
The richest star system among the more familiar stars is Castor: the 24th brightest star in the sky and an intrinsically sextuple system. Unlike our Sun, which is the only star in our system, practically half of all stars have one or more companions in their stellar systems.
Credit: NASA

Surveying nearby stars reveals that 48% of them are bound in multi-star systems.

Tarantula Nebula JWST stars
The central concentration of this young star cluster found in the heart of the Tarantula Nebula is known as R136, and contains many of the most massive stars known. Among them is R136a1, which comes in at about ~260 solar masses and shines brighter than more than 8 million suns, making it the heaviest known star. Although great numbers of cooler, redder stars are also present, the brightest, bluest ones dominate this image, although they have the shortest lifetime, living for between 1-10 million years only. Within a cloud of gas, the process of core fragmentation leads to enormous populations of large numbers of stars.
Credit: NASA, ESA, CSA, STScI, Webb ERO Production Team

But what about the heaviest, most massive stars of all?

This fragment of the young star-forming region NGC 2014 showcases many stars that are bluer, more massive, and much shorter lived than our Sun. However, the fainter, redder, less luminous stars are far more numerous, making us wonder just what “typical” truly is for a star. NGC 2014 is also found in the Large Magellanic Cloud: over 160,000 light-years away.
Credit: NASA, ESA and STScI

They’re too short-lived to perform an accurate census of them.

flame nebula infrared spitzer
The Flame Nebula, shown here in a combination of X-ray data (from Chandra) and infrared light (from Spitzer), showcases a young, massive star cluster at the center, which carves out a spectacular shape in the surrounding gaseous material that was used for star-formation. Direct observations of the hottest, brightest, most massive stars that form inside these regions are difficult, as there are frequently large amounts of (visible) light-blocking matter intervening. After only a few million years, the star(s) primarily responsible for illuminating the Flame Nebula will all have died away.
Credit: X-ray: NASA/CXC/PSU/K.Getman, E.Feigelson, M.Kuhn & the MYStIX team; Infrared: NASA/JPL-Caltech

The environments in which they form, star-forming regions, are often opaque.

A group of satellite dishes under a starry sky, capturing celestial signals from star birth.
The Atacama Large Millimetre/submillimetre Array, or ALMA, is the most powerful, highest-resolution array of radio telescopes in the world. Although it only has the light-gathering power of all its dishes, combined, it has the resolution of the space between the dishes, making it capable of resolving details no other observatory can see.
Credit: ESO/B. Tafreshi (TWANight.org)

But ALMA, the Atacama Large Millimetre-submillimetre Array, can finally peer inside.

VLBI technique
In very-long baseline interferometry (VLBI), the radio signals are recorded at each of the individual telescopes before being shipped to a central location. Each data point that’s received is stamped with an extremely accurate, high-frequency atomic clock alongside the data in order to help scientists get the synchronization of the observations correct. For optical interferometry, rather than radio data, the various optical beams must be combined on-site, as otherwise the necessary amplitude and phase information about the electromagnetic signals will not be adequately preserved.
Credit: public domain/Rnt20 at English Wikipedia

This array of radio telescopes, through interferometry, performs an incredible trick.

Black hole jet shadow M87
This image of the black hole, event horizon, and beginning of the launched jet comes from a 6.5 billion solar mass black hole at the center of galaxy Messier 87 (M87). The radio astronomy technique of very-long baseline interferometry was essential to the construction of each aspect of this detailed image.
Credit: R.-S. Lu (SHAO), E. Ros (MPIfR), S. Dagnello (NRAO/AUI/NSF)

It gathers light with only the individual dishes, but its resolving power covers the space between them.

alma central core gas SN 1987a
In the center of the remnant of SN 1987A, ALMA, with its incredible resolution and long-wavelength capabilities, was able to observe a particularly hot spot within the gas and dust of SN 1987A. The extra heat is thought by many to be an indicator of a young neutron star, which would make this the youngest neutron star ever discovered.
Credit: P. Cigan et al./Cardiff University

As a result, it can image at higher resolution than any other observatory.

Alma low resolution captures star birth.
At “low” resolution, ALMA, observing the protostar cluster G333.23-0.06, can pick out dense cores of matter and identify regions where various new stars are in the process of forming. Even though these regions cannot be identified at optical wavelengths, ALMA’s high resolution and radio-wavelength sensitivity make it ideal for this task.
Credit: S. Li et al., Nature Astronomy, 2024

Recently, ALMA observed the high-mass stellar protocluster G333.23-0.06.

        Description: A series of images showcasing star birth and various types of stars.
The dense cores of protostar cluster G333.23–0.06, as identified by ALMA, show strong evidence for large levels of multiplicity within these cores. Binary cores are common, and groups of multiple binaries, forming quaternary systems, are also quite common. Triplet and quintuplet systems are also found inside, while, for these high-mass clumps, singlet stars turn out to be quite rare.
Credit: S. Li et al., Nature Astronomy, 2024

Singlet stars were rare, but binaries were overwhelmingly common.

A plot illustrating the diversity of stars through star birth processes.
If the kinetic energy of members of the same star system is below the gravitational energy, systems can be considered to be gravitationally bound, and the systems within G333.23–0.06 where that is determined to be the case are shown here. Particularly at the high-mass end, for stars of 5 solar masses and up, multi-star systems are overwhelmingly not just common, but perhaps even the norm.
Credit: S. Li et al., Nature Astronomy, 2024

Triplet, quadruplet, and even quintuplet systems were spotted directly.

A map displaying the locations of various star birth areas.
In this close-up view of a dense core of matter in the star-forming region G333.23–0.06, a variety of dense clumps of matter have been probed with ALMA, revealing (insets, going clockwise from top left) a quaternary, binary, quintuple, and triplet star system within it. This indicates these star systems were born as multiples, rather than capturing other members later on.
Credit: S. Li, MPIA / J. Neidel, MPIA Graphics Department; Data: ALMA Observatory

The lack of a disk suggests core fragmentation as the formation mechanism.

Nasa's spacecraft explores star birth.
This ALMA observation of a high-mass protostar cluster, G351.77-0.54, has gotten down to ~120 AU spatial resolution, corresponding to 0.06 arc-seconds at the distance of these protostars. The gaseous material is fragmenting into at least four separate cores, a hint (now with further evidence) that core fragmentation, rather than anything having to do with a disk, is a major player in determining how many stars form in these high-mass star-forming regions. When nuclear fusion reactions initiate inside these protostar cores, they will officially become full-fledged stars.
Credit: H. Beuther et al., Astronomy & Astrophysics, 2017

Like dogs, not humans, a single, high-mass stellar “child” is rare.

yellowballs star formation spitzer
Three separate regions illustrate various stages of a newly forming star’s life, which are totally obscured in the optical and can only be seen in the infrared. At left, a protostar emits radiation that’s shrouded in light-blocking dust. In the center, a ‘yellowball’ announces the start of nuclear fusion, but still cannot be seen in the optical due to all the surrounding matter. At right, a more evolved star has begun to blow an ionized bubble in the surrounding region. For high-mass stars, we now know that forming a singlet system, as opposed to a multi-star system, is a relative rarity.
Credit: NASA/JPL-Caltech

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