"Long before it's in the papers"
November 02, 2012

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Astronomers tally leftover light from dead, living stars

Nov. 2, 2012
Courtesy of NASA
and World Science staff

As­tro­no­mers say they have made the most ac­cu­rate tally of all the star­light that has ev­er shown—in­clud­ing light from long-dead stars.

This light keeps trav­el­ing through the uni­verse “even af­ter the stars cease to shine, and this cre­ates a fos­sil radia­t­ion field,” said lead sci­ent­ist Mar­co Ajello of Stan­ford Uni­vers­ity in Cal­i­for­nia the Uni­vers­ity of Cal­i­for­nia at Berke­ley.

The find­ing, made us­ing da­ta from NASA’s Fer­mi Gamma-ray Space Tel­e­scope, is ex­pected to help as­tro­no­mers un­der­stand the ear­li­est pe­ri­od of star forma­t­ion.

This diagram (click here for en­larged ver­sion) shows how the Fer­mi in­stru­ment (low­er right) col­lects gam­ma-rays from bla­zars at dif­fer­ent points in cos­mic his­to­ry. The "tube" rep­re­sents a very sim­pli­fied il­lus­tra­tion of cos­mic his­to­ry from the birth of the uni­verse in the Big Bang 13.7 bil­lion years ago (up­per left) to to­day (low­er right.) Star form­a­tion peaked when the uni­verse was about 3 bil­lion years old and has been de­clin­ing ever since, ast­ron­om­ers say. (Cred­it: NA­SA's God­dard Space Flight Cen­ter)

Gamma-rays are the most en­er­get­ic form of light. The Fer­mi in­stru­ment ob­serves the en­tire sky in high-energy gam­ma-rays ev­ery three hours, cre­at­ing the most de­tailed known map of how the sky looks in gam­ma-rays.

The to­tal sum of star­light is called the ex­tra­ga­lac­tic back­ground light, and con­sists of light par­t­i­cles, or pho­tons, that are zip­ping around space, as­tro­no­mers say. In a sense it is a vast cloud of par­t­i­cles. Ac­cord­ingly it acts as a sort of fog, by dim­ming our view of gam­ma-rays head­ed for Earth. 

Sci­en­tists say the fog’s thick­ness can be es­ti­mat­ed from the ex­tent of this dim­ming. Ajello and his team did so by stu­dy­ing gam­ma-rays from 150 bla­zars, or ga­lax­ies that are blast­ing gamma-rays di­rectly to­ward us be­cause of gi­ant black holes at their cores.

A black hole is an ob­ject so com­pact that its gra­vity be­comes over­pow­ering, and an­y­thing that strays with­in a cer­tain dis­tance of it falls in­to it. But shortly be­fore reach­ing the bound­a­ry zone where this fate ir­re­trievably oc­curs, for rea­sons not fully un­der­stood, some of the ap­proach­ing ma­te­ri­al can in­stead get blast­ed out of its gal­axy in the form of a radia­t­ion je­t. When this je­t hap­pens to point to­ward Earth, the gal­axy looks es­pe­cially bright to us and is called a blazar.

Gamma-rays from blazar je­ts trav­el bil­lions of years to reach Earth. In that time, they go through the dim­ming “fog” of the back­ground star­light. What hap­pens is that a gamma-ray can col­lide with a par­t­i­cle of star­light, caus­ing the gamma-ray to be lost to our view. The long­er the trip, the more gamma-ray par­t­i­cles are knocked out. Thus, more dis­tant blazars show few­er high-energy gamma-rays; the fur­thest show al­most none.

The re­search­ers de­ter­mined the av­er­age gamma-ray damp­ing ef­fect across three dis­tance ranges be­tween 9.6 bil­lion years ago and to­day. From this, they es­ti­mat­ed the fog’s thick­ness. To ac­count for the ob­serva­t­ions, they said, the av­er­age stel­lar dens­ity in the cos­mos is about 1.4 stars per 100 bil­lion cu­bic light-years, which means the av­er­age dis­tance be­tween stars in the uni­verse is about 4,150 light-years. A light-year is the dis­tance light trav­els in a year.

A pape­r de­scrib­ing the find­ings was pub­lished Nov. 1 in the on­line edi­tion of the re­search jour­nal Sci­ence.

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Astronomers say they have made the most accurate tally of all the starlight that has ever shown—including light from long-dead stars. This light keeps traveling through the universe “even after the stars cease to shine, and this creates a fossil radiation field,” said lead scientist Marco Ajello of Stanford University in California the University of California at Berkeley. The finding, made using data from NASA’s Fermi Gamma-ray Space Telescope, is expected to help astronomers understand the earliest period of star formation. Gamma-rays are the most energetic form of light. The Fermi instrument observes the entire sky in high-energy gamma-rays every three hours, creating the most detailed known map of how the cosmos looks in gamma-rays. The total sum of starlight is called the extragalactic background light, and consists of light particles, or photons, that are zipping around space, astronomers say. In a sense it is a vast cloud of particles. Accordingly it acts as a sort of fog, by dimming our view of gamma-rays headed for Earth. Scientists say the fog’s thickness can be estimated from the extent of this dimming. Ajello and his team did so by studying gamma-rays from 150 blazars, or galaxies that are blasting gamma-rays directly toward us because of the action of giant black holes at their cores. A black hole is an object so compact that its gravity becomes overpowering, and anything that strays within a certain distance of it falls into it. But shortly before reaching the boundary zone where this fate irretrievably occurs, for reasons not fully understood, some of the approaching material can instead get blasted out of its galaxy in the form of a radiation jet. When this jet happens to point toward Earth, the galaxy looks especially bright to us and is called a blazar. Gamma-rays from blazar jets travel billions of years to reach Earth. In that time, they through the dimming “fog” of the extragalactic background light. What happens is that a gamma-ray can collide with a particle of starlight, causing the gamma-ray to be lost to our view. The longer the trip, the more gamma-ray particles are knocked out. Thus, more distant blazars show fewer high-energy gamma-rays; the furthest show almost none. The researchers determined the average gamma-ray damping effect across three distance ranges between 9.6 billion years ago and today. From this, they estimated the fog’s thickness. To account for the observations, they said, the average stellar density in the cosmos is about 1.4 stars per 100 billion cubic light-years, which means the average distance between stars in the universe is about 4,150 light-years. A light-year is the distance light travels in a year. A paper describing the findings was published Nov. 1 in the online edition of the research journal Science.