Astronomy ABC - H for Herschel Space Observatory
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 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 cold — the coldest places on Earth are balmy in comparison. Yet however cold it is, anything warmer than its surroundings radiates energy in the form of infrared radiation. Suitable telescopes can detect it. Such observations help us to understand phenomena such as the birth and evolution of stars.
Back to the spectrum diagram, it shows the infrared spectrum bordering 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.) The water vapor in our atmosphere absorbs much of the infrared radiation, so 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 a 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. We now know it as infrared, though at that time no one was aware of invisible radiation.
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 a mission expected to last 3.5 years lasted for several bonus months.
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:
- formation and evolution of galaxies
- star formation and how it interacts with the material between the stars
- the atmospheres and surfaces of bodies in the Solar System
- 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 provided a database for further research.
The header image shows a mosaic of observations of the Taurus Molecular Cloud carried out by Herschel. It's where a myriad of stars are being born, Credit: ESA/Herschel/NASA/JPL-Caltech
The usual targets for an infrared telescope aren't stars, but rather the material between the stars. 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: R Gendler; ESA/Herschel/PACS/SPIRE/J. Fritz;ESA/XMM-Newton/EPIC/W. Pietsch
When Herschel peered into the Universe, it went back in time. 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, a galaxy that's 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 of 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.
When the coolant ran out in April 2013, Herschel's instruments couldn't work properly. Although data collection was over, there was still data to process and refine. That part of the mission continued into 2017.
It's fitting that this great telescope honored two astronomers who helped to lay the foundations of modern astronomy.
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