The Tarantula Nebula, also called 30 Doradus, is the brightest star-forming region in our part of the galaxy. It’s in the Large Magellanic Cloud (LMC) and contains the most massive and hottest stars we know of. The Tarantula Nebula has been a repeat target for the Hubble since the telescope’s early years.
Astronomers originally thought that just one massive star cluster shone brightly in a huge star forming region of the Tarantula Nebula, also known as 30 Doradus. But closer analysis using data from the Hubble Space Telescope shows that it is actually two different clusters that are just starting to collide and merge. A team of astronomers led by Elena Sabbi of the Space Telescope Science Institute noticed that different stars in the same region were of different ages, by at least one million years. Besides the age differences, the scientists also noticed two distinct regions, with one having the elongated “look” of a merging cluster.
“Stars are supposed to form in clusters,” said Sabbi, “but there are many young stars outside 30 Doradus that could not have formed where they are; they may have been ejected at very high velocity from 30 Doradus itself.”
Sabbi and her team were initially looking for runaway stars — fast-moving stars that have been kicked out of their stellar nurseries where they first formed.
But they noticed something unusual about the cluster when looking at the distribution of the low-mass stars detected by Hubble. It is not spherical, as was expected, but has features somewhat similar to the shape of two merging galaxies where their shapes are elongated by the tidal pull of gravity.
Some models predict that giant gas clouds out of which star clusters form may fragment into smaller pieces. Once these small pieces precipitate stars, they might then interact and merge to become a bigger system. This interaction is what Sabbi and her team think they are observing in 30 Doradus.
There are also an unusually large number of runaway, high-velocity stars around 30 Doradus, and after looking more closely at the clusters, the astronomers believe that these runaway stars were expelled from the core of 30 Doradus as the result of the dynamical interactions between the two star clusters. These interactions are very common during a process called core collapse, in which more-massive stars sink to the center of a cluster by dynamical interactions with lower-mass stars. When many massive stars have reached the core, the core becomes unstable and these massive stars start ejecting each other from the cluster.
The big cluster R136 in the center of the 30 Doradus region is too young to have already experienced a core collapse. However, since in smaller systems the core collapse is much faster, the large number of runaway stars that has been found in the 30 Doradus region can be better explained if a small cluster has merged into R136.
The entire 30 Doradus complex has been an active star-forming region for 25 million years, and it is currently unknown how much longer this region can continue creating new stars. Smaller systems that merge into larger ones could help to explain the origin of some of the largest known star clusters, Sabbi and her team said.
Follow-up studies will look at the area in more detail and on a larger scale to see if any more clusters might be interacting with the ones observed. In particular the infrared sensitivity of NASA’s planned James Webb Space Telescope (JWST) will allow astronomers to look deep into the regions of the Tarantula Nebula that are obscured in visible-light photographs. In these areas cooler and dimmer stars are hidden from view inside cocoons of dust. Webb will better reveal the underlying population of stars in the nebula.
The 30 Doradus Nebula is particularly interesting to astronomers because it is a good example of how star-forming regions in the young universe may have looked. This discovery could help scientists understand the details of cluster formation and how stars formed in the early Universe.
Happy birthday to the Hubble Space Telescope! On April 24, 1990, HST was launched into low Earth orbit. Now, nearly 22 years later, Hubble is still producing incredible, stunning images of the farthest reaches of the Universe. For this year’s anniversary, the Hubble team took a special panoramic view of 30 Doradus, a raucous stellar breeding ground, located in the heart of the Tarantula nebula. The image comprises one of the largest mosaics ever assembled from Hubble photos and consists of observations taken by Hubble’s Wide Field Camera 3 and Advanced Camera for Surveys, combined with observations from the European Southern Observatory’s MPG/ESO 2.2-metre telescope that trace the location of glowing hydrogen and oxygen.
The Tarantula nebula is 170,000 light-years away in the Large Magellanic Cloud, a small, satellite galaxy of our Milky Way. No known star-forming region in our galaxy is as large or as prolific as 30 Doradus.
The stars in this image add up to a total mass millions of times bigger than that of our Sun. The image is roughly 650 light-years across and contains some rambunctious stars, from one of the fastest rotating stars to the speediest and most massive runaway star.
The nebula is close enough to Earth that Hubble can resolve individual stars, giving astronomers important information about the stars’ birth and evolution. Many small galaxies have more spectacular starbursts, but the Large Magellanic Cloud’s 30 Doradus is one of the only star-forming regions that astronomers can study in detail. The star-birthing frenzy in 30 Doradus may be partly fueled by its close proximity to its companion galaxy, the Small Magellanic Cloud.
The image reveals the stages of star birth, from embryonic stars a few thousand years old still wrapped in dark cocoons of dust and gas to behemoths that die young in supernova explosions. 30 Doradus is a star-forming factory, churning out stars at a furious pace over millions of years. The Hubble image shows star clusters of various ages, from about 2 million to about 25 million years old.
The image was made from 30 separate fields, 15 from each camera. Hubble made the observations in October 2011. Both cameras were making observations at the same time.
Take an interactive tour of the Tarantula Nebula at the HubbleSite
In an astronomical version of “Biggest Loser” meets “Survivor,” a heavy weight star has been kicked out of its stellar nursery. This huge runaway star is rushing away from its birthplace at more than 402,336 kilometers per hour (250,000 miles an hour), and it likely was ejected by a group of even larger sibling stars. The future outlook for this tough-luck star seemingly doesn’t improve: Paul Crowther of the University of Sheffield, a member of the team who made the observations of 30 Dor #016, said the wayward star will continue to streak across space and will eventually end its life in a titanic supernova explosion, likely leaving behind a remnant black hole. There’s a new reality series in there somewhere!
The star on the run is found 375 light-years from its suspected home, a giant star cluster called R136 in 30 Doradus, also called the Tarantula Nebula, about roughly 170,000 light-years from Earth. R136 contains several stars topping 100 solar masses each. 30 Dor #016 is 90 times more massive than our Sun.
Astronomers say runaway stars can be made in a couple of ways: a star may encounter one or two heavier siblings in a massive, dense cluster and get booted out through a stellar game of pinball. Or, a star may get a ‘kick’ from a supernova explosion in a binary system, with the more massive star exploding first.
“It is generally accepted, however, that R136 is sufficiently young, 1 million to 2 million years old, that the cluster’s most massive stars have not yet exploded as supernovae,” says COS team member Danny Lennon of the Space Telescope Science Institute. “This implies that the star must have been ejected through dynamical interaction.”
The renegade star may not be the only runaway in the region. Two other extremely hot, massive stars have been spotted beyond the edges of 30 Doradus. Astronomers suspect that these stars, too, may have been ejected from their home. They plan to analyze the stars in detail to determine whether 30 Doradus might be unleashing a barrage of massive stellar runaways into the surrounding neighborhood.
The observations came from a team-effort using Hubble’s newly installed Cosmic Origins Spectrograph (COS) to take an image of the region in 2009, an optical image of the star taken by the Wide Field Planetary Camera 2 in 1995, and another spectroscopic study from the European Southern Observatory’s Very Large Telescope (VLT) at the Paranal Observatory. It was first observed in 2006 when a team led by Ian Howarth of University College London spotted it with the Anglo-Australian Telescope at Siding Spring Observatory.
COS’s ultraviolet spectroscopic observations showed that the wayward star is unleashing a fury of charged particles in one of the most powerful stellar winds known, a clear sign that it is extremely massive, perhaps as much as 90 times heavier than the Sun. The star, therefore, also must be very young, about 1 million to 2 million years old, because extremely massive stars live only a few million years.
The VLT observations revealed that the star’s velocity is constant and not a result of orbital motion in a binary system. Its velocity corresponds to an unusual motion relative to the star’s surroundings, evidence that it is a runaway star.
The study also confirmed that the light from the runaway is from a single massive star rather than the combined light of two lower-mass stars. In addition, the observation established that the star is about 10 times hotter than the Sun, a temperature that is consistent with a high-mass object.
“These results are of great interest because such dynamical processes in very dense, massive clusters have been predicted theoretically for some time, but this is the first direct observation of the process in such a region,” says Nolan Walborn of the Space Telescope Science Institute in Baltimore and a member of the COS team that observed the misfit star. “Less massive runaway stars from the much smaller Orion Nebula Cluster were first found over half a century ago, but this is the first potential confirmation of more recent predictions applying to the most massive young clusters.”
The research team, led by Chris Evans of the Royal Observatory Edinburgh, published the study’s results May 5 in the online edition of The Astrophysical Journal Letters.