Matthew McCrory, Francesca Valsecchi, and Vicky Kalogera  /
The massive black hole in M33 X-7 is hidden in the X-ray bright center of the pancake-like accretion disk of matter. The black-hole's hot (blue) and massive star companion is losing mass in a wind that gets pulled and captured by the black hole.
updated 10/20/2010 3:27:15 PM ET 2010-10-20T19:27:15

A massive black hole that is spewing X-rays and locked in a tight orbital dance around a huge, dim star finally has a good origins story.

Unlike binary set-ups that result when a giant star absorbs mass from a companion star that has nearly exhausted its nuclear fuel, the huge black hole M33 X-7 could have formed because, in this case, the companion still had plenty of hydrogen to burn, according to a new study.

The enormous stellar black hole has 15.7 times the mass of the sun and is orbiting an even larger star which is 70 times the sun's mass once every 3.45 days. Other X-ray binaries typically have stellar black holes of 10 solar masses.

The pair is located about 2.7 million light-years from Earth in the galaxy Messier 33.

Researchers had been hard-pressed to come up with a satisfying explanation for M33 X-7 using existing models of binary X-ray systems.

"This massive black hole is orbiting the most massive star ever discovered in this class of systems, and the orbit is very tight compared to the size of the star," study leader Francesca Valsecchi of Northwestern University told [ Gallery: Black holes of the universe ]

As a further wrinkle, the companion star is much dimmer than usual for its mass.

How can this exist?

In one model proposed to explain M33 X-7, after the primary star begins to run out of hydrogen fuel and expand, its outer region forms an envelope that encompassed both it and its companion.

But for a star big enough to generate a nearly 16 solar mass black hole, as in this case, the envelope would have led to the merger of the two stars.

While some other proposed models can account for the masses and tight orbit of the system, they don't solve the mystery of the black hole's X-ray glow and spin. The relative dimness of the black hole's star companion and their elliptical orbit also were unexplained.

Valsecchi and her colleagues came up with a different possible history for the pair.

Black hole's new origins story

In their model, the black hole-star pairing stems from a giant star the future black hole nearly 100 times the mass of our sun, circling a second star of about 30 solar masses every three days or so.

  1. Most popular

In such a tight orbit, the future black hole is able to start transferring mass while it is still burning hydrogen into helium. As a result, it loses most of its hydrogen envelope (becoming a so-called Wolf-Rayet star) and sheds the rest of the envelope in the form of stellar wind, exposing its helium core.

Its companion grows far more massive in the process, becoming the larger and more massive of the two stars. But it remains dim because the added mass doesn't dramatically change the rate of nuclear reactions in its core, researchers said.

Finally the progenitor star collapses under its own gravity, yielding a black hole, and begins absorbing stellar wind from its companion, leading to powerful X-ray emission. Energy released during the gravitational collapse imparts a kick to the black hole that leads to an elliptical orbit, and the black hole's spin results from the spin of the star itself.

"It is encouraging to have confirmation of our basic understanding of binary evolution and black hole formation," Valsecchi said, "as it allows us to trust our physical models and make predictions for other black hole systems that are yet to be discovered."

According to the researchers, existing Wolf-Rayet binaries containing massive companion stars may represent the early stages of process that led to M33 X-7, providing further evidence for the model.

Valsecchi said she and her colleagues are currently investigating the history of another X-ray binary system that contains the most massive black hole to have formed around another star.

© 2013 All rights reserved. More from

Photos: Month in Space: January 2014

loading photos...
  1. Southern stargazing

    Stars, galaxies and nebulas dot the skies over the European Southern Observatory's La Silla Paranal Observatory in Chile, in a picture released on Jan. 7. This image also shows three of the four movable units that feed light into the Very Large Telescope Interferometer, the world's most advanced optical instrument. Combining to form one larger telescope, they are greater than the sum of their parts: They reveal details that would otherwise be visible only through a telescope as large as the distance between them. (Y. Beletsky / ESO) Back to slideshow navigation
  2. A balloon's view

    Cameras captured the Grandville High School RoboDawgs' balloon floating through Earth's upper atmosphere during its ascent on Dec. 28, 2013. The Grandville RoboDawgs’ first winter balloon launch reached an estimated altitude of 130,000 feet, or about 25 miles, according to coaches Mike Evele and Doug Hepfer. It skyrocketed past the team’s previous 100,000-feet record set in June. The RoboDawgs started with just one robotics team in 1998, but they've grown to support more than 30 teams at public schools in Grandville, Mich. (Kyle Moroney / AP) Back to slideshow navigation
  3. Spacemen at work

    Russian cosmonauts Oleg Kotov, right, and Sergey Ryazanskiy perform maintenance on the International Space Station on Jan. 27. During the six-hour, eight-minute spacewalk, Kotov and Ryazanskiy completed the installation of a pair of high-fidelity cameras that experienced connectivity issues during a Dec. 27 spacewalk. The cosmonauts also retrieved scientific gear outside the station's Russian segment. (NASA) Back to slideshow navigation
  4. Special delivery

    The International Space Station's Canadian-built robotic arm moves toward Orbital Sciences Corp.'s Cygnus autonomous cargo craft as it approaches the station for a Jan. 12 delivery. The mountains below are the southwestern Alps. (NASA) Back to slideshow navigation
  5. Accidental art

    A piece of art? A time-lapse photo? A flickering light show? At first glance, this image looks nothing like the images we're used to seeing from the Hubble Space Telescope. But it's a genuine Hubble frame that was released on Jan. 27. Hubble's team suspects that the telescope's Fine Guidance System locked onto a bad guide star, potentially a double star or binary. This caused an error in the tracking system, resulting in a remarkable picture of brightly colored stellar streaks. The prominent red streaks are from stars in the globular cluster NGC 288. (NASA / ESA) Back to slideshow navigation
  6. Supersonic test flight

    A camera looking back over Virgin Galactic's SpaceShipTwo's fuselage shows the rocket burn with a Mojave Desert vista in the background during a test flight of the rocket plane on Jan. 10. Cameras were mounted on the exterior of SpaceShipTwo as well as its carrier airplane, WhiteKnightTwo, to monitor the rocket engine's performance. The test was aimed at setting the stage for honest-to-goodness flights into outer space later this year, and eventual commercial space tours.

    More about SpaceShipTwo on PhotoBlog (Virgin Galactic) Back to slideshow navigation
  7. Red lagoon

    The VLT Survey Telescope at the European Southern Observatory's Paranal Observatory in Chile captured this richly detailed new image of the Lagoon Nebula, released on Jan. 22. This giant cloud of gas and dust is creating intensely bright young stars, and is home to young stellar clusters. This image is a tiny part of just one of 11 public surveys of the sky now in progress using ESO telescopes. (ESO/VPHAS team) Back to slideshow navigation
  8. Fire on the mountain

    This image provided by NASA shows a satellite view of smoke from the Colby Fire, taken by the Multi-angle Imaging SpectroRadiometer aboard NASA's Terra spacecraft as it passed over Southern California on Jan. 16. The fire burned more than 1,863 acres and forced the evacuation of 3,700 people. (NASA via AP) Back to slideshow navigation
  9. Where stars are born

    An image captured by NASA's Spitzer Space Telescope shows the Orion Nebula, an immense stellar nursery some 1,500 light-years away. This false-color infrared view, released on Jan. 15, spans about 40 light-years across the region. The brightest portion of the nebula is centered on Orion's young, massive, hot stars, known as the Trapezium Cluster. But Spitzer also can detect stars still in the process of formation, seen here in red hues. (NASA / JPL-Caltech) Back to slideshow navigation
  10. Cygnus takes flight

    Orbital Sciences Corp.'s Antares rocket rises from NASA's Wallops Flight Facility on Wallops Island, Va, on Jan. 9. The rocket sent Orbital's Cygnus cargo capsule on its first official resupply mission to the International Space Station. (Chris Perry / NASA) Back to slideshow navigation
  11. A long, long time ago...

    This long-exposure picture from the Hubble Space Telescope, released Jan. 8, is the deepest image ever made of any cluster of galaxies. The cluster known as Abell 2744 appears in the foreground. It contains several hundred galaxies as they looked 3.5 billion years ago. Abell 2744 acts as a gravitational lens to warp space, brightening and magnifying images of nearly 3,000 distant background galaxies. The more distant galaxies appear as they did more than 12 billion years ago, not long after the Big Bang. (NASA / NASA via AFP - Getty Images) Back to slideshow navigation
  12. Frosty halo

    Sun dogs are bright spots that appear in the sky around the sun when light is refracted through ice crystals in the atmosphere. These sun dogs appeared on Jan. 5 amid brutally cold temperatures along Highway 83, north of Bismarck, N.D. The temperature was about 22 degrees below zero Fahrenheit, with a 50-below-zero wind chill.

    Slideshow: The Year in Space (Brian Peterson / The Bismarck Tribune via AP) Back to slideshow navigation
  1. Editor's note:
    This image contains graphic content that some viewers may find disturbing.

    Click to view the image, or use the buttons above to navigate away.

  2. Editor's note:
    This image contains graphic content that some viewers may find disturbing.

    Click to view the image, or use the buttons above to navigate away.

  3. Editor's note:
    This image contains graphic content that some viewers may find disturbing.

    Click to view the image, or use the buttons above to navigate away.

  4. Editor's note:
    This image contains graphic content that some viewers may find disturbing.

    Click to view the image, or use the buttons above to navigate away.


Discussion comments


Most active discussions

  1. votes comments
  2. votes comments
  3. votes comments
  4. votes comments