Massive Stars: Can They Form on Their Own? — Dare to Know
Massive stars produce the heavier elements from which we’re all made. Find out how the Hubble Telescope is shedding new light on where they came from.
In an earlier story, we talked about the stellar nursery in the Orion Nebula. We said that it was the most prominent example of the places where stars form in our night sky.
We should have pointed out that this is only true in the Northern Hemisphere. Below the equator, there’s another stellar nursery with an equally fascinating story to tell.
I’ve never seen the Tarantula Nebula with my own eyes because I’ve never been to the Southern Hemisphere. If I’d grown up there, no doubt I’d have spent time gazing at two bright nebulae between the constellations Dorado and Mensa.
TARANTULA NEBULA IN THE SOUTHERN HEMISPHERE
They’re at least as easy to see as the Orion Nebula I saw in the winter sky as a boy. Those of us with European ancestors think of these phenomena as somehow modern, but that’s just our bias.
Of course, they’ve been in the sky just as long as the objects we’re familiar with, and we Europeans didn’t “discover” them any more than we discovered Australia or Brazil. The first scientist to document them was Persian astronomer Abd al-Rahman al-Sufi in 964 A.D.
The reason we Europeans have a vague impression of discovering these bright nebulae is that the explorers Americo Vespucci and especially Ferdinand Magellan spread the news about them when they got back to Europe from their voyages. Today we call them the Large and Small Magellanic Clouds after Magellan.
WE CALL THEM THE LARGE AND SMALL MAGELLANIC CLOUDS
Despite their names, they’re not clouds or even nebulae. We now know that they’re galaxies. This story concerns the Large Magellanic Cloud.
The Lage Magellanic Cloud is a galaxy but a very tiny one, being about 1% of the mass of the Milky Way Galaxy. Astronomers classify it as a dwarf, irregular galaxy. It’s irregular shape comes from having had run-ins with some other galaxies in the distant past.
For many years, scientists have thought that it orbits our galaxy in much the same way that the moon orbits the Earth. That’s being disputed lately because of the speed it’s going.
GALAXY ABOUT 1% THE MASS OF THE MILKY WAY
The Large Magellanic Cloud is about 170,000 light-years away. In about 2.4 billion years, it will collide with our galaxy.
Satellite or not, it’s part of what astronomers call the Local Group. That also includes Andromeda, the Triangulum Galaxy and about 30 others.
There’s a genuine nebula inside the Large Magellanic Cloud. Scientists named it the Tarantula Nebula, but as with a lot of star lore, it takes some imagination to recognize the spider or its legs.
STAR CLUSTER CONTAINING NINE ENORMOUS YOUNG STARS
What we can easily see, especially with the aid of the Hubble Telescope, is a star cluster called R136 containing nine enormous, young stars. These stars are all more than 100 times the mass of the sun and they shine about 30 million times brighter.
Massive stars like this have a different life cycle than our sun. They seem to be Neil Young fans, believing that “it’s better to burn out than to fade away.”
A typical massive star will only live a few million years versus our sun, which will last a total of nearly 10 billion years. Their brief lives end in a spectacular explosion called a supernova.
PERFECT LABORATORY FOR STUDYING STAR FORMATION
The Tarantula Nebula contains the largest sample of very massive stars scientists have identified to date. They find it the perfect laboratory for studying star formation.
Stars are born out of clouds of gas and dust like the Tarantula Nebula. Some areas of nebulae are more clumpy than others and these clumps collapse under their own gravity.
Stars form as the clump’s core collapses. The infant star start to spin, and this motion creates a disk out of the remaining gas and dust that becomes their solar system.
STARS HAVE MASSES 150 TIMES THAT OF THE SUN
Until about 2010, astronomers believed that the upper limit for the size of a star was about 100 stellar masses. Then, ten years ago, a team of researchers led by Professor Paul Crowther of the University of Sheffield showed that stars were forming in the Tarantula Nebula whose mass was more than 150 times that of the sun.
Since then, the debate has been whether these massive stars form via the usual process or by some other means. One hypothesis is that they result from two very large binary stars merging together.
Massive stars give off most of their energy in the ultra-violet range. So, the team used the Hubble Telescope’s Space Telescope Imaging Spectrograph (STIS) to try to distinguish what was going on in the Tarantula Nebula with higher resolution.
SPACE TELESCOPE IMAGING SPECTROGRAPH
The results show that at least some massive stars form through the usual process. Professor Saida Caballero-Nieves is a co-author of the study, and he explains it this way.
“From what we know about the frequency of massive mergers, this scenario can’t account for all the really massive stars that we see in R136, so it would appear that such stars can originate from the star formation process.”
Another debate concerns whether massive stars form within clusters like other stars, or whether they can form in isolation. Astronomers had observed what appeared to be very isolated massive stars, which seemed to suggest that they might be able to form individually.
“ISOLATED” STARS SURROUNDED BY HUNDREDS OF INFANT STARS
This new study shows that these stars aren’t actually isolated at all. With higher ultra-violet resolution, it’s clear that they are all surrounded by hundreds of pre-main-sequence (newborn) stars.
These results seem to suggest that all of the categories of stars in the universe form by much the same process. Even so, this is far from settled.
The debate now is about whether all of the newborn stars are genuine. It’s very hard to distinguish between a clump that is becoming a star and one that is fated to remain a clump of dust.
MASSIVE STARS ARE THE SOURCE OF HEAVY ELEMENTS
It’s important to understand the origin and life cycle of massive stars because they are the source of most of the heavy elements in the universe. Carl Sagan famously said that we are all “made of star-stuff.”
He was referring to the fact that massive stars produce and release the heavier elements of which we are made when their lives end in supernoavae. How massive stars are born is a gap in the science-based story we all need to be able tell each other about our place in the universe.
As the European Space Agency/Hubble partnership explains, “Only detailed analysis and observations will reveal their true nature and that will help to finally solve the unanswered question of the origin of massive stars.”
We always have more to learn if we dare to know.
NASA/ESA Hubble Space Telescope Partnership
Stellar Clusterings Around “Isolated” Massive Young Solar Objects in the Large Magellanic Cloud
The R136 star cluster dissected with Hubble Space Telescope/STIS. I. Far-ultraviolet spectroscopic census and the origin of He IIλ1640 in young star clusters
Newborn Stars Bringing Forth Solar Systems
Where Do Heavy Metals Come From? (Not Ozzy Osbourne!)Where Do Heavy Metals Come From? Encore Set!
Originally published at https://daretoknow.ca on March 30, 2020.