When Is a Star Not a Star?

When it’s a brown dwarf — but where do we draw the line?

Often called “failed stars,” brown dwarfs are curious cosmic creatures. They’re kind of like swollen, super-dense Jupiters, containing huge amounts of matter yet not quite enough to begin fusing hydrogen in their cores. Still, there has to be some sort of specific tipping point, and astronomers (being the scientists that they are) would like to know: when does a brown dwarf stop and a star begin?

Researchers from Georgia State University now have the answer.

From a press release issued Dec. 9 from the National Optical Astronomy Observatory (NOAO):

For most of their lives, stars obey a relationship referred to as the main sequence, a relation between luminosity and temperature – which is also a relationship between luminosity and radius. Stars behave like balloons in the sense that adding material to the star causes its radius to increase: in a star the material is the element hydrogen, rather than air which is added to a balloon. Brown dwarfs, on the other hand, are described by different physical laws (referred to as electron degeneracy pressure) than stars and have the opposite behavior. The inner layers of a brown dwarf work much like a spring mattress: adding additional weight on them causes them to shrink. Therefore brown dwarfs actually decrease in size with increasing mass.

Read more: The Secret Origin Story of Brown Dwarfs

As Dr. Sergio Dieterich, the lead author, explained, “In order to distinguish stars from brown dwarfs we measured the light from each object thought to lie close to the stellar/brown dwarf boundary. We also carefully measured the distances to each object. We could then calculate their temperatures and radii using basic physical laws, and found the location of the smallest objects we observed (see the attached illustration, based on a figure in the publication). We see that radius decreases with decreasing temperature, as expected for stars, until we reach a temperature of about 2100K. There we see a gap with no objects, and then the radius starts to increase with decreasing temperature, as we expect for brown dwarfs. “

Dr. Todd Henry, another author, said: “We can now point to a temperature (2100K), radius (8.7% that of our Sun), and luminosity (1/8000 of the Sun) and say ‘the main sequence ends there’ and we can identify a particular star (with the designation 2MASS J0513-1403) as a representative of the smallest stars.”

The relation between size and temperature at the point where stars end and brown dwarfs begin (based on a figure from the publication) Image credit: P. Marenfeld & NOAO/AURA/NSF.
The relation between size and temperature at the point where stars end and brown dwarfs begin (based on a figure from the publication) Image credit: P. Marenfeld & NOAO/AURA/NSF.

“We can now point to a temperature (2100K), radius (8.7% that of our Sun), and luminosity (1/8000 of the Sun) and say ‘the main sequence ends there’.”

Dr. Todd Henry, RECONS Director

Aside from answering a fundamental question in stellar astrophysics about the cool end of the main sequence, the discovery has significant implications in the search for life in the universe. Because brown dwarfs cool on a time scale of only millions of years, planets around brown dwarfs are poor candidates for habitability, whereas very low mass stars provide constant warmth and a low ultraviolet radiation environment for billions of years. Knowing the temperature where the stars end and the brown dwarfs begin should help astronomers decide which objects are candidates for hosting habitable planets.

The data came from the SOAR (SOuthern Astrophysical Research) 4.1-m telescope and the SMARTS (Small and Moderate Aperture Research Telescope System) 0.9-m telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile.

Read more here.

The Secret of the Stars

“Say, do you like mystery stories? Well we have one for you. The concept: relativity.

Well look at that, it’s a new video from John D. Boswell — aka melodysheep — which goes into autotuned detail about one of the standard principles of astrophysics, Einstein’s theory of general relativity.

Featuring clips from Michio Kaku, Brian Cox, Neil deGrasse Tyson, Brian Greene and Lisa Randall, I’d say E=mc(awesome).

John has been entertaining science fans with his Symphony of Science mixes since 2009, when his first video in the series — “A Glorious Dawn” featuring Carl Sagan — was released. Now John’s videos are eagerly anticipated by fans (like me) who follow him on YouTube and on Twitter as @musicalscience.

E = mc2… that is the engine that lights up the stars.”

(What does Einstein’s famous mass-energy equivalence equation mean? For a brief and basic explanation, check out the American Museum of Natural History’s page here.)

Astronomy Cast Ep. 269: Mass

Last week we talked about energy, and this week we’ll talk about mass. And here’s the crazy thing. Mass, matter, the stuff that the Universe is made of, is the same thing as energy. They’re connected through Einstein’s famous formula – E=mc2. But what is mass, how do we measure it, and how does it become energy, and vice versa.

Click here to download the episode.

Or subscribe to: astronomycast.com/podcast.xml with your podcatching software.

“Mass” on the Astronomy Cast website, with shownotes and transcript.

And the podcast is also available as a video, as Fraser and Pamela now record Astronomy Cast as part of a Google+ Hangout:

Continue reading “Astronomy Cast Ep. 269: Mass”

Atomic Mass Unit

Faraday's Constant

Believe it or not, there are actually several atomic mass units … however, the one that’s standard – throughout chemistry, physics, biology, etc – is the unified atomic mass unit (symbol u). It is defined as 1/12 (one-twelfth) of the mass of an isolated carbon-12 atom, in its ground state, at rest. You’ll still sometime see the symbol amu – which stands for atomic mass unit – but that’s actually two, slightly different, units (and each is different from the unified atomic mass unit!) … these older units are defined in terms of oxygen (1/16th of an isolated oxygen-16 atom, and 1/16th of an ‘average’ oxygen atom).

As it’s a unit of mass, the atomic mass unit (u) should also have a value, in kilograms, right? It does … 1.660 538 782(83) x 10-27 kg. How was this conversion worked out? After all, the kilogram is defined in terms of a bar of platinum-iridium alloy, sitting in a vault in Paris! First, it is important to recognize that the unified atomic mass unit is not an SI unit, but one that is accepted for use with the SI. Second, the kilogram and unified atomic mass unit are related via a primary SI unit, the mole, which is defined as “the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12“. Do you remember how many atoms there are in a mole of an element? Avogadro’s number! So, work out the Avogadro constant, and the conversion factor follows by a simple calculation …

The Dalton (symbol D, or Da) is the same as the unified atomic mass unit … why have two units then?!? In microbiology and biochemistry, many molecules have hundreds, or thousands, of constituent atoms, so it’s convenient to state their masses in terms of ‘thousands of unified atomic mass units’. That’s far too big a mouthful, so convention is to use kDa (kilodaltons).

Find out more on the (unified) atomic mass unit, from the Argonne National Laboratory, from the International Union of Pure and Applied Chemistry, and from the National Institute of Standards and Technology (NIST).

Learning to Breathe Mars Air and Mini-Detector Could Find Life on Mars or Anthrax at the Airport are two Universe Today articles relevant to the atomic mass unit.

Energy Levels and Spectra and Inside the Atom are two Astronomy Cast episodes related to the atomic mass unit.

Newton Ask a Scientist
Wise Geek

Earth’s Mass

The Earth’s mass is 5.9736 x 1024 kg. That’s a big number, so let’s write it out in full: 5,973,600,000,000,000,000,000,000 kg. You could also say the Earth’s mass is 5.9 sextillion tonnes. Phew, that’s a lot of mass.

That sounds like a lot, and it is, but the Earth has a fraction of the mass of some other objects in the Solar System. The Sun has 333,000 times more mass than the Earth. And Jupiter has 318 times more mass. But then there are some less massive objects too. Mars has only 11% the mass of the Earth.

Because of its high mass for its size, Earth actually has the highest density of all the planets in the Solar System. The density of Earth is 5.52 grams per cubic centimeter. The high density comes from the Earth’s metallic core, which is surrounded by the rocky mantle. Less dense planets, like Jupiter, are just made up of gases like hydrogen.

We’ve written several articles about the mass of planets in the Solar System. Here’s an article about the mass of Mercury, and here’s an article about the mass of the Sun.

If you’d like more information on the Earth mass, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded an episode of Astronomy Cast all about the Earth. Listen here, Episode 51: Earth.

How Many Atoms Are There in the Universe?

It’s no secret that the universe is an extremely vast place. That which we can observe (aka. “the known Universe”) is estimated to span roughly  93 billion light years. That’s a pretty impressive number, especially when you consider its only what we’ve observed so far. And given the sheer volume of that space, one would expect that the amount of matter contained within would be similarly impressive.

But interestingly enough, it is when you look at that matter on the smallest of scales that the numbers become the most mind-boggling. For example, it is believed that between 120 to 300 sextillion (that’s 1.2 x 10²³ to 3.0 x 10²³) stars exist within our observable universe. But looking closer, at the atomic scale, the numbers get even more inconceivable.

At this level, it is estimated that the there are between 1078 to 1082 atoms in the known, observable universe. In layman’s terms, that works out to between ten quadrillion vigintillion and one-hundred thousand quadrillion vigintillion atoms.

And yet, those numbers don’t accurately reflect how much matter the universe may truly house. As stated already, this estimate accounts only for the observable universe which reaches 46 billion light years in any direction, and is based on where the expansion of space has taken the most distant objects observed.

The history of theA billion years after the big bang, hydrogen atoms were mysteriously torn apart into a soup of ions.universe starting the with the Big Bang. Image credit: grandunificationtheory.com
The history of the universe starting the with the Big Bang. Image credit: grandunificationtheory.com

While a German supercomputer recently ran a simulation and estimated that around 500 billion galaxies exist within range of observation, a more conservative estimate places the number at around 300 billion. Since the number of stars in a galaxy can run up to 400 billion, then the total number of stars may very well be around 1.2×1023  – or just over 100 sextillion.

On average, each star can weigh about 1035 grams. Thus, the total mass would be about 1058 grams (that’s 1.0 x 1052 metric tons). Since each gram of matter is known to have about 1024 protons, or about the same number of hydrogen atoms (since one hydrogen atom has only one proton), then the total number of hydrogen atoms would be roughly 1086 – aka. one-hundred thousand quadrillion vigintillion.

Within this observable universe, this matter is spread homogeneously throughout space, at least when averaged over distances longer than 300 million light-years. On smaller scales, however, matter is observed to form into the clumps of hierarchically-organized luminous matter that we are all familiar with.

In short, most atoms are condensed into stars, most stars are condensed into galaxies, most galaxies into clusters, most clusters into superclusters and, finally, into the largest-scale structures like the Great Wall of galaxies (aka. the Sloan Great Wall). On a smaller scale, these clumps are permeated by clouds of dust particles, gas clouds, asteroids, and other small clumps of stellar matter.

Representation of the timeline of the universe over 13.7 billion years, and the expansion in the universe that followed. Credit: NASA/WMAP Science Team.
Representation of the timeline of the universe over 13.7 billion years, and the expansion in the universe that followed. Credit: NASA/WMAP Science Team.

The observable matter of the Universe is also spread isotropically; meaning that no direction of observation seems different from any other and each region of the sky has roughly the same content. The Universe is also bathed in a wave of highly isotropic microwave radiation that corresponds to a thermal equilibrium of roughly 2.725 kelvin (just above Absolute Zero).

The hypothesis that the large-scale universe is homogeneous and isotropic is known as the cosmological principle. This states that physical laws act uniformly throughout the universe and should, therefore, produce no observable irregularities in the large scale structure. This theory has been backed up by astronomical observations which have helped to chart the evolution of the structure of the universe since it was initially laid down by the Big Bang.

The current consensus amongst scientists is that the vast majority of matter was created in this event, and that the expansion of the Universe since has not added new matter to the equation. Rather, it is believed that what has been taking place for the past 13.7 billion years has simply been an expansion or dispersion of the masses that were initially created. That is, no amount of matter that wasn’t there in the beginning has been added during this expansion.

However, Einstein’s  equivalence of mass and energy presents a slight complication to this theory. This is a consequence arising out of Special Relativity, in which the addition of energy to an object increases its mass incrementally. Between all the fusions and fissions, atoms are regularly converted from particles to energies and back again.

Atom density is greater at left (the beginning of the experiment) than 80 milliseconds after the simulated Big Bang. Credit: Chen-Lung Hung
Atom density is greater at left (the beginning of the experiment) than 80 milliseconds after the simulated Big Bang. Credit: Chen-Lung Hung

Nevertheless, observed on a large-scale, the overall matter density of the universe remains the same over time. The present density of the observable universe is estimated to be very low – roughly 9.9 × 10-30 grams per cubic centimeter. This mass-energy appears to consist of 68.3% dark energy, 26.8% dark matter and just 4.9% ordinary (luminous) matter. Thus the density of atoms is on the order of a single hydrogen atom for every four cubic meters of volume.

The properties of dark energy and dark matter are largely unknown, and could be uniformly distributed or organized in clumps like normal matter. However, it is believed that dark matter gravitates as ordinary matter does, and thus works to slow the expansion of the Universe. By contrast, dark energy accelerates its expansion.

Once again, this number is just a rough estimate. When used to estimate the total mass of the Universe, it often falls short of what other estimates predict. And in the end, what we see is just a smaller fraction of the whole.

We’ve got a many articles that are related to the amount of matter in the Universe here in Universe Today, like How Many Galaxies in the Universe, and How Many Stars are in the Milky Way?

NASA also has the following articles on the universe, like How many galaxies are there? and this article on the Stars in Our Galaxy.

We also have podcast episodes from Astronomy Cast on the subject of Galaxies and Variable Stars.