Exploring Our Galaxy’s Ancient Brown Dwarfs

As the name implies, a brown dwarf is small… only about 7% the size of the Sun. As far as stellar senior citizens go, they’re cool. Zipping along through space at speeds of 100 to 200 kilometers per second, they may have formed back when our galaxy was young – perhaps 10 billion years ago. Now a team of astronomers headed by Dr. David Pinfield at the University of Hertfordshire has identified a pair of the oldest brown dwarfs known… a set of orbs which could be the harbinger of a huge amount of new, unseen objects.

Although we sometimes refer to them as stars, brown dwarfs are in a class of their own. Because they didn’t ignite in nuclear fusion, they don’t generate internal heat like an ordinary star. After they are formed, they continue to cool and fade as time passes. This process makes them very difficult to observe and the discovery of two very old brown dwarfs, with temperatures of 250-600 C is cause for astronomical excitement.

Just how did Pinfield’s team pick such tiny objects out of the vastness of space? The discovery was facilitated thanks to a survey made by the Wide-field Infrared Survey Explorer (WISE), a NASA observatory that scanned the mid-infrared sky from orbit in 2010 and 2011. The ancient objects are cataloged as WISE 0013+0634 and WISE 0833+0052, and they are located in the constellations of Pisces and Hydra. Because they are so elusive, they were also confirmed by large ground-based telescopes (Magellan, Gemini, VISTA and UKIRT).

However, identifying the pair wasn’t easy. Seeing through the eyes of infrared reveals a crowded space – one populated with reddened stars, distant background galaxies and pockets of nebulous gas and dust. Picking out such a small character from a stellar cast would be like finding one tiny pearl in the vastness of an ocean. But Pinfield’s researchers employed a new method which utilizes WISE’s capabilities. As it scanned the sky over and over again, it revealed the cool, brown dwarfs – picking up the faint signature that other searches had missed.

These two particular brown dwarfs are different from the other slow movers of their type. By studying their spectra, the astronomers have identified atmospheres almost entirely comprised of hydrogen. This sets them apart from younger stars which have an abundance of heavier elements. Does being lighter make them speedier? According to Pinfield, “Unlike in other walks of life, the galaxy’s oldest members move much faster than its younger population.”

Stars near to Sun are considered the “local volume” and are created with three overlapping populations – the thin disk, the thick disk and the halo. Each of these layers has a certain amount of age associated with it: the oldest being the thickest and its member stars move up and down at a higher rate of speed. The halo contains both disks, along with the initial materials which formed the very first stars. Thin disk objects abound in the local volume and account for about 97% of the local stars, while thick disk and halo objects are a meager 3%. Chances are, brown dwarfs belong to that smaller percentage which explains why these fast-moving thick-disk/halo objects are only now being revealed.

Just how many may await discovery? Scientists surmise there may be as many as 70 billion brown dwarfs in the galaxy’s thin disk, and the thick disk and halo take up significantly larger galactic volumes. Even at a tiny 3%, this means there could be an army of ancient brown dwarfs in the galaxy. “These two brown dwarfs may be the tip of an iceberg and are an intriguing piece of astronomical archaeology,” said Pinfield. “We have only been able to find these objects by searching for the faintest and coolest things possible with WISE. And by finding more of them we will gain insight into the earliest epoch of the history of the galaxy.”

Original Story Source: Royal Astronomical Society News Release. For further study: “A deep WISE search for very late type objects and the discovery of two halo/thick-disk T dwarfs: WISE 0013+0634 and WISE 0833+0052”, D. J. Pinfield et al, Monthly Notices of the Royal Astronomical Society, in press.

10 Replies to “Exploring Our Galaxy’s Ancient Brown Dwarfs”

    1. I’ve deleted your insulting reply because it was in violation of this site’s comment policy.

      FYI, 200 km is 124.275 miles, not “100 miles” – if you’re gonna insult someone, then first make sure that you get your own damn facts right!

      Furthermore, I’ve checked your profile: In just over one month of joining Disqus, you’ve already received 13 flags elsewhere for making just 9 comments, and now you’ve come over here. You’re just a bloody troll – and a 3rd rate novice one!

      Therefore, since you have nothing constructive to offer in the Universe Today comments, I have now blacklisted you.

  1. With the addition of Brown Dwarfs to the growing collection of Red Dwarfs, even if we were to somehow achieve a method of traveling at a percentage of light speed, there would be another problem…. KER SPLAT! Imagine the difficulty in trying to avoid them at relativistic speeds.

    1. 33 found Brown dwarfs within 26ly, and none as Close as the closest starsystem (alpha centauri).
      Maybe we have different definitions of ‘near the solarsystem’, or do you have some special implication to your statement?

      1. no implication intended or made. it is not often that these objects are stated as being “near our solar system”. you need not remind me of how vast the universe is.

  2. Is there a way brown dwarfs could figure in how gas giant planets could have migrated so close to their primaries?

  3. Old, cold brown dwarfs could be suspected to have very little turbulence in their interior. If that is the case, there should be a deep upper atmosphere consisting primarily by hydrogen, which could explain the observation that 2 of them seems to be primarily made by hydrogen – only the hydrogen being detectable at this point.
    The interior is likely making up for the difference.

  4. Are there any Brown Dwarf stars sandwiched between our Sun and WR 104? Since Brown Dwarf stars are huge reservoirs of Hydrogen (between 14 to 75 Jupiter masses) they have the capacity to absorb the radiation from a GRB. We’re also protected by the Oort Cloud, which theoretically consists of billions of icy/rocky objects. The Hydrogen in the ice and the denser elements in the rocks will absorb the radiation also.

    But just to be safe, the Astronauts should program a cadre of Robots to carve out a settlement inside Olympus Mons, and dig tunnels to connect all 20 inactive volcanoes on Mars. Think of volcanoes as instant fortresses, protecting its inhabitants from the elements–an iron mountain of sorts.

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