The discovery of a runaway star in Hubble's image of the Orion Nebula suggests a stellar tussle ejected three stars 540 years ago.

Orion Nebula
Hubble re-imaged the Orion Nebula using its Wide Field Camera 3 in 2015.
NASA / ESA / M. Robberto (STScI / ESA) / Hubble Space Telescope Orion Treasury Project Team

Within the bedlam that is the Orion Nebula, newborn stars fling about heated and ionized gas, creating an appearance of turbulent chaos. Yet the stars themselves generally move with slow dignity across the field. The Hubble Space Telescope revealed these stars' steady proper motions across the sky in two images, one taken in 1998 using the NICMOS camera and the other in 2015 using the Wide Field Camera 3 (pictured here).

But there are exceptions. Two stars discovered decades ago, the infrared-radiating Becklin-Neugebauer (BN) object and a radio emitter dubbed Source I, are outwinging their protostellar neighbors in Orion at 60,000 mph and 22,000 mph, respectively. They're zooming in opposite directions from the centrally located Kleinmann-Low Nebula.

Now, Kevin Luhman (Penn State University) and colleagues report in the March 20th Astrophysical Journal Letters (full text here), new examinations of the Hubble images have revealed a third star, "Source x," moving at more than 120,000 mph across the sky, apparently from the same origin point. (Luhman was originally looking for rogue planets, but the discovery of a runaway star didn't disappoint!)

Source x's proper motion
This video shows the proper motion of Source x across the sky over a 17-year period.
NASA / ESA / K. Luhman (Penn State University)

The astronomers collected an infrared spectrum of the object, which shows that the protostar weighs in at 2 or 3 solar masses, lower than its runaway companions. (BN is probably 20 solar masses and Source I is 7 solar masses). The three may have been part of a single multi-star system — but they would have parted ways 540 years ago.

Detailed view of Kleinmann-Low Nebula
The insets show the current positions of three wayward stars in the Kleinmann-Low Nebula, as well as their starting point 540 years ago (green x). (Note that the infrared image does not capture Source I, whose position from radio images is marked with a red circle.) The second inset demonstrates the proper motion of Source x.
NASA / ESA / K. Luhman (Penn State University) / M. Robberto (STScI)

Source x is the missing link to this equation. While astronomers had already suspected that the Becklin-Neugebauer object and Source I had been involved in some kind of stellar tussle centuries ago, the system's energy didn't add up. The new measurement of Source x's proper motion accounts for the missing energy and seals the deal: all three stars were originally part of the same system. (In this scenario, the radio emission from Source I may be coming from two sources in a single tight binary or from a stellar merger.)

Stellar dynamics
This artist's conception demonstrates a four-star system, where the stars all orbit one another. Such a system is chaotic and it's possible that as two members move closer together to form a tight binary or merge (producing Source I), in the process ejecting the other two members (BN and Source x).
NASA / ESA / Z. Levy (STScI)

Source x may clear up the runaway stars' origin, but it raises another question of its own. Along with the other stars in this stellar mashup, it is young enough to still be enshrouded in a dusty disk. But how did the stars retain (or reform) their disks as they were ejected? The answer may need to wait for Hubble's succcessor, the James Webb Space Telescope, scheduled to launch in October 2018.

To read more about this system and find high-resolution images, fly over to the Hubble team's press release.


Image of Graham-Wolf


March 20, 2017 at 7:14 pm

Thank you, Monica, for an outstanding report.

Great diagrams and vids.
A real eye-opener.
I'll never be able to look at M42/NGC 1976 with a straight face again.
Whoever claimed that astronomy was dull, boring and un-exciting, really needs to read your report.

That'll certainly sober them up, in short order.
There's heaps of data lying out there, still waiting to be mined.
Keep up the great work, and keeping us firmly up-to-date..
Most heartening to see women out there in front, going hard... just like the late (and great) Vera Rubin! And Maria Mitchell at Vassar.

Regards from 46 South, NZ.
Graham W. Wolf (proud astro-feminist).

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Image of Anthony Barreiro

Anthony Barreiro

March 20, 2017 at 7:26 pm

The third paragraph says "(full text here)", but my browser doesn't show an active link.

And by the way, 60,000 miles per hour equals about 27,000 meters/second
22,000 mph = ~10,000 m/s
120,000 mph = ~54,000 m/s

The ratios between these speeds are the same regardless of units, but, still, SI, right?

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