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.
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.
Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
Humanity once thought our Solar System was typical.
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. The sizes, masses, and distances between the various objects is not generally appreciated, as these scales are far outside our everyday experience.
The other stars, presumably, were Sun-like objects, but very far away.
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.
We soon learned that stars and stellar systems varied tremendously.
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 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.
While our Solar System has just one star, half of all stellar systems have multiples.
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.
The image shows the central region of the Tarantula Nebula in the Large Magellanic Cloud. The young and dense star cluster R136 can be seen at the lower right of the image. The tidal forces exerted on the Large Magellanic Cloud by the Milky Way are triggering a wave of star-formation in there, and likely the reverse is true as well.
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.
They’re too short-lived to perform an accurate census of them.
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.
The environments in which they form, star-forming regions, are often opaque.
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.
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.
This array of radio telescopes, through interferometry, performs an incredible trick.
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.
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.
As a result, it can image at higher resolution than any other observatory.
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.
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. It is expected that the stars forming in nebulae all throughout the Universe, including in the Eagle Nebula, have similar clumpy properties.
Singlet stars were rare, but binaries were overwhelmingly common.
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.
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
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
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.