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Astronomy ABC - H for Herschel Space Observatory

The biggest telescope ever sent into space was the Herschel Space Observatory. It was an infrared telescope named for William and Caroline Herschel. But what do we mean by infrared, who launched the telescope, and what did we learn from it?

Infrared and the electromagnetic spectrum
Light waves carry energy. The ones we can see, known as visible light or the optical spectrum, form a small part of the electromagnetic spectrum. The diagram shows that shorter waves have higher frequencies, i.e., there are more waves per second. The shortest waves have the most energy, and are emitted at high temperatures in violent processes such as hot gas circling black holes.

High-energy events are dramatic, but most of the Universe is very cold the coldest places on Earth are balmy in comparison. Yet however cold it is, if something is warmer than its surroundings, it radiates energy in the form of infrared radiation. This can be detected by suitable telescopes. Observations of this kind help us to understand phenomena such as the birth and evolution of stars.

The infrared spectrum borders the red light of the visible spectrum. The near infrared is close enough to visible frequencies for most major optical telescopes to detect. However seeing in the far infrared and submillimeter needs dedicated infrared telescopes. (Submillimeter refers to a narrow band of electromagnetic waves between the infrared and microwaves.) Since the water vapor in our atmosphere absorbs much of the infrared radiation, Earth isn't a good place for observing. Nonetheless telescopes in dry places on high mountains, and high-altitude balloons and aircraft have provided data. Infrared space telescopes can study objects impossible to see from Earth.

Herschel Space Observatory
When the European Space Agency (ESA) planned their space telescope, they called it FIRST (Far Infrared and Submillimeter Telescope), but later settled on "Herschel". When William Herschel (1738-1822) was studying the spectrum of the Sun, he realized that something he couldn't see made the temperature rise on his thermometer. At the time no one was aware of invisible radiation, but we now know this as infrared.

The Herschel Space Observatory's 3.5-meter (11.5-foot) mirror reflected the light onto three instruments that detected the far infrared and submillimeter. The detectors had to be cooled to an extremely low temperature to keep their heat from swamping the signal. They used liquid helium for this.

ESA launched the Herschel telescope in 2009, and the mission was expected to last 3.5 years, but it lasted for several bonus months.

Herschel's mission
ESA set out four primary objectives for Herschel. They related to phenomena for which Herschel's high resolution in the far infrared and submillimeter would yield data that were previously unobtainable. They were:
  1. formation and evolution of galaxies

  2. star formation and how it interacts with the material between the stars

  3. the atmospheres and surfaces of bodies in the Solar System

  4. molecular chemistry in the universe

What Herschel saw
Herschel made over 35,000 scientific observations during the mission. There were many discoveries, and it's also providing a database for further research.

The usual targets for an infrared telescope aren't the stars. The material between the stars is what's of interest, as it shows structure that can't be seen in visible light. For example, here is an example of how different wavelengths show us different things. It's the Andromeda Galaxy in the far infrared and the optical, with a composite in the middle. In the optical we see the stars, and in the infrared we see the glowing dust lanes, the brightest bits being where they're heated by hot young stars. (Image credit: Robert Gendler (visible); ESA/Herschel/SPIRE/HELGA (far-infrared))

When Herschel peered deep into the Universe, it was going back in time. There's a speed limit, even on traveling light, so when we see an object a billion light years away, we're seeing it as it was a billion years ago. One of Herschel's discoveries was an exceptionally active starburst galaxy. That's a galaxy which is producing a great mass of new stars. Not only was it highly active, but astronomers saw it as it was just 880 million years after the Big Bang when the Universe was less than ten percent its present age.

Is there a Goldilocks mass for efficient galaxy formation? Analysis of Herschel data says yes. If a galaxy is too small, there isn't enough matter for more than one generation of star formation. Yet if it has too much matter, the gas cools so slowly that it doesn't collapse into stars.

But the observations weren't all of galaxies far, far away. Measurements of Comet Hartley 2 suggested that comets might have brought water to Earth. Also that Comet Shoemaker-Levy 9 had brought water to Jupiter when it collided with the giant planet. Does that mean only comets brought water to the planets? Herschel detected water vapor on the dwarf planet Ceres, so maybe asteroid collisions were also important.

The end
When the coolant ran out in April 2013, Herschel's instruments couldn't work properly. Although it was finally the end of the data collection, there was still data to process and refine. That part of the mission continued into 2017.

The Herschel space telescope took us back in time billions of years and gave us a better view of our own Solar System. It's fitting that it honored two astronomers who helped to lay the foundations of modern astronomy.
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