Why can't we see quasars directly?

Astronomy: No understanding of quasars

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If you look at a quasar with a telescope in the visible spectral range, it hardly differs from a conventional star. With radio telescopes, however, it becomes apparent that these mysterious objects are primarily radio sources. But it wasn't until exactly 50 years ago - in 1963 - astronomers published the first measurement of the distance to such a celestial object, did it become clear how enormous the luminosity of the quasars actually is: They are billions of light years away from Earth.

Quasars are exceptionally bright. They emit as much energy as thousands of giant galaxies - and that from a region as tiny as the solar system. They cover the entire electromagnetic spectrum, from radio waves to gamma rays. Many of them also fire bundled particle beams into space at almost the speed of light. As a result, huge, club-shaped particle clouds or bubbles form that extend over millions of light years and emit radio waves.

This article is contained in Spectrum - Die Woche, 11. KW 2013

Before the first distance measurement, astronomy was limited to only being able to explore the local, well-known universe. The distant universe, on the other hand, seemed out of reach. But now the light emanating from distant quasars enabled us to look into the cosmic past. In the 1920s, astronomers discovered that the universe was expanding. The cosmos must therefore have a finite age - it is currently estimated at around 13.7 billion years. And so it was a great pleasure for astronomers to track down ever more distant quasars and thus to get closer and closer to the Big Bang with ever better technology. But how quasars emit the enormous amounts of energy is not yet understood in detail.

Quasars and other active galactic nuclei (AGNs) get their energy from the accretion of gas and stars on supermassive black holes, which are hidden in galaxy centers. So far everyone agrees. But the details are still in the dark (see the "Quasar engines" diagram). Many theoretical models of quasars have little or no predictive power and, accordingly, of little use in my opinion. The task now is to recognize this problem and, above all, to focus now on understanding the physics of black holes.


The first known objects far outside the Milky Way were radio galaxies, whose club-shaped emission regions were discovered in the 1950s with the help of radio telescopes. With optical telescopes, astronomers were finally able to determine the distance to the galaxies: Due to the expansion of the universe, the spectral lines of chemical elements such as hydrogen shift to longer wavelengths on their way to Earth. This redshift turns out to be stronger the further away the emitting galaxy is from us. Until 1960, the radio galaxy 3C295 (the 295th object in the third Cambridge catalog of radio sources) held the record with a redshift of 0.464 - the light began its journey 4.8 billion years ago. Unfortunately, radio galaxies are faint and difficult to see from a distance.

With the first known redshift for a quasar, the cosmic horizon expanded suddenly in March 1963. In the journal "Nature" Maarten Schmidt described the bright source 3C273 - another object from the Cambridge catalog - and determined its redshift to a value of 0.158, corresponding to a distance of 2.4 billion light years [1]. The spectrum of 3C273 deviates from that of an ordinary star, Bev Oke showed in an article published at the same time [2]. The astonishing brightness paired with the great distance of the source indicated an enormously high energy release. Significantly higher redshifts for other quasars soon followed.