## December 25, 2006

### The Earliest Stars?

#### Posted by John Baez

Executive summary: using delicate techniques to carefully sift through the infrared background radiation, the authors claim to find radiation not accounted for by previously known sources. Assuming the now-standard ΛCDM cosmology, the sources of this radiation date back to less than 1 billion years after the Big Bang, and were individually much brighter than current-day stars.

Here’s a picture of the data:

On top is a photograph taken by the Spitzer Space Telescope: a 10-hour infrared exposure of a tiny patch of sky, 6 × 12 arcminutes across, chosen for having a bare minimum of foreground stars, galaxies and dust. (For comparison, the Moon is 30 arcminutes across.) On the bottom is the same picture with known sources of infrared subtracted. What’s left may be the severely redshifted light from early stars!

Or, it may not. Ned Wright of UCLA has said “I’m very skeptical of this result. I think it’s wrong. I think what they’re seeing is incompletely subtracted residuals from nearby sources.”

The first stars play an interesting role in the history of the Universe. Roughly 380,000 years after the Big Bang, the hot hydrogen and helium forming our Universe cooled down to 3000 kelvin — cool enough to no longer be ionized, so light could freely pass through. The period from then until the first stars formed is called the “Dark Ages”. During this era, gas cooled down and clumped under its own gravity — perhaps with a lot of help from dark matter.

In keeping with their name, the Dark Ages are still shrouded in mystery. Which formed first — stars or galaxies? And, when did the Dark Ages end? It’s currently believed that the first stars formed sometime between 150 million and 1 billion years after the Big Bang. At the later end of that range, the Universe could have gotten quite cold before starlight warmed up the interstellar gas and reionized it.

There’s even a spooky theory that the Universe was full of hydrogen snowflakes near the end of the Dark Ages. But, the current best guess, based on WMAP data, says that reionization happened 400 million years after the Big Bang. This would too early for hydrogen snow, since my rough calculation says the microwave background radiation was 30 kelvin then, while hydrogen freezes at 14 kelvin.

The first stars are believed to have been very large: perhaps hundreds of times the size of our Sun! But, these so-called “Population III” stars have not actually been seen — hence the fuss surrounding Kashlinsky et al’s paper. When these stars died and exploded, they began the first stage of a process, still underway, in which the interstellar gas of the Universe became ever more rich in heavier elements.

The Universe is a fascinating place. Merry Christmas!

Posted at December 25, 2006 12:31 AM UTC

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### Re: The Earliest Stars?

Speaking of the Dark Ages — I have a little timeline of the history of the universe, and it has an embarrassingly large gap between 13.3 billion years ago (“reionization” - when the first stars heated and reionized the hydrogen gas forming most of our universe at the time) and 4.55 billion years ago (the formation of the Sun).

Does anyone know some dates of exciting events between these two times? I can’t believe it was a completely boring stretch…

The formation of the Milky Way?

The first galaxies?

The first supernovae?

The first quasars?

The first population II stars?

The first population I stars?

Other stuff?

(By the way, I don’t actually believe we know the two dates listed above as accurately as I’ve quoted them! People use these figures, but I wouldn’t be surprised if the last one or two decimals change in the future.)

Posted by: John Baez on December 25, 2006 4:49 AM | Permalink | Reply to this

### Re: The Earliest Stars?

I can’t believe it was a completely boring stretch…

… it’s a matter of scale how long it stretches… try log

Posted by: Bee on December 27, 2006 3:11 PM | Permalink | Reply to this

### Re: The Earliest Stars?

Bee wrote:

… it’s a matter of scale how long it stretches… try log.

Yeah. Ironically, it seems useful to work both with

$log (t - t_{Big Bang})$

and with

$log (t_{now} - t)$

In other words, the exciting events in the history of the Universe seem to bunch up near the Big Bang and also near now. You can see this clearly on my timeline.

Of course, this is why people speak of a Technological Singularity in analogy to the Initial Singularity… a term which has led to much controversy.

But, I don’t want to talk about that stuff now — it’s been beaten to death. I want to know what were the most interesting events between reionization and the formation of the Sun! I can’t believe the Universe spent 8 billion years lazing around with nothing to show for itself.

When did these things happen:

The formation of the Milky Way?

The first galaxies?

The first supernovae?

The first quasars?

The first population II stars?

The first population I stars?

Clearly a bunch of people are goofing off enjoying their holidays when they should be helping me out with these questions… it’s disgraceful.

Posted by: John Baez on December 27, 2006 3:52 PM | Permalink | Reply to this

### Re: The Earliest Stars?

John wrote:

Clearly a bunch of people are goofing off enjoying their holidays when they should be helping me out with these questions… it’s disgraceful.

Hooray for those who blog over Christmas!

Posted by: Tom Leinster on December 27, 2006 4:41 PM | Permalink | Reply to this

### Re: The Earliest Stars?

Ironically, it seems useful to work both with

(1)$\log ( t - t _ BigBang )$

and with

(2)$\log ( t _ now - t )$

In other words, the exciting events in the history of the Universe seem to bunch up near the Big Bang and also near now.

This makes perfect sense to me. I would only add that the events bunched up towards the Big Bang are exciting in a more absolute sense than the events bunched up near now. In another 10 billion years, the events near now will seem quite insignificant to our intellectual descendants (assuming for the sake of argument that there are some), which the events bunched up near the Big Bang (as long as we aren’t completely on the wrong track about what that is) will still seem exciting to them.

Posted by: Toby Bartels on December 28, 2006 5:32 AM | Permalink | Reply to this

### Future cosmic Timeline; Re: The Earliest Stars?

Had to think logarithmically to write this

TIMELINE COSMIC FUTURE

Posted by: Jonathan Vos Post on December 29, 2006 7:43 AM | Permalink | Reply to this

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