CTIO at sunset (NOAO/AURA/NSF)

HOW DO WE STUDY STARS? Electromagnetic Radiation (X-ray, UV, Optical, IR, Microwave, Radio) Particle Emission (Neutrinos, Stellar Winds, Cosmic Rays, ...) Gravitational Wave Radiation ...

Stars are distant which makes ordinary experiments impossible. The nearest star is Proxima Centauri at a distance of 4.3 light years ~ 40 trillion kilometers. (One light year is the distance a beam of light travels in 1 year ~ 9.3 trillion kilometers.) The Earth is ~ 8.3 light minutes from the Sun. The moon is ~ 1.3 light seconds from the Earth. Traveling at the speed of the typical interplanetary probe, ~ 20 kilometers per second, it would take 70,000 years to reach Proxima Centauri!

We cannot control stars, in particular, we cannot design the types of experiments we would like to perform on stars; We take what nature gives to us, that is, for the most part we passively observe the sky.

HOW DO WE OBSERVE STARS (Chapter 6)?

Electromagnetic Radiation

Most observations of Celestial Objects are made using Optical Telescopes located at ground-based (e.g., Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, National Solar Observatory, Mauna Kea Observatory, ...) and space-based orbiting observatories (Space Telescope Science Institute). Optical light will be defined in a second. For now, take it to mean the type of light to which our eyes are sensitive. The essential advantages of telescopes are essentially to make a huge eye to allow us: (1) to collect more EM radiation, see fainter objects; and (2) to resolve smaller angles, see smaller features and objects.

A ton of information about the Universe has been gleaned from optical observations, however, much more can be learned if we consider more than just optical light [objects in the Universe produce many other forms of radiation]. Collectively, the overall light, radiation, phenomenon is referred to as Electromagnetic radiation (EM radiation). Until recently the complete EM spectrum was not utilized because most types of EM radiation do not penetrate the Earth's atmosphere and so do not reach the surface of the Earth.

The major windows fall in the optical (visual portion) of the spectrum and in the microwave and radio portion of the spectrum. There are also windows in the Infrared, IR. Fortunately for us, the high energy part of the EM spetrum, the gamma-ray, x-ray, and most of the ultraviolet, UV is blocked by the atmosphere of the Earth shielding us from these forms of high-energy electromagnetic radiation.

Today, because we can place telescopes into orbit about the Earth, (1) we are able to study stars across many portions of the EM spectrum and: (2) to get above the blurring effects of the atmosphere (the effects of seeing; effects on dog star, Sirius) (see section 5.4, pp. 122-125).

The most spectacular of these missions has been the James Webb Telescope (JWST) (see Discovery 5-1, pp. 114-115),

Webb Space Telescope (JWST) The primary mirror of JWST is 6.5 meters across composed of of 18 gold-plated hexagonal deployable segments. The JWST deployable sunishield is the size of a tennis court. JWST has four science instruments, the Near-Infrared Camera (NIRCam), the Near-Infrared Spectrograph (NIRSpec), the Mid-Infrared Instrument (MIRI), and the Near-Infrared Imager and Slitless Spectrograph (NIRISS) with the Fine Guidance Sensor (FGS). JWST, unlike HST covers not only the optical (visible) portion of the spectrum, it also reaches into the Near Infrared, and Mid Infrared (0.6-28.5 micrometers). JWST is located 1.5 million kilometers from Earth at what is known as the second Lagrange point, L2. Compared to terrestrial telescopes, JWST (and HST) are large but not the largest telescpes. The largest Terrestrial telescopes are 10 meters in diameter. JWST (and HST) are so valuable because: They are above the atmosphere of the Earth and can observe into the ultraviolet, UV, to the near- and mid-Infrared (IR). The latter is crucial for studying objects when the Universe was wrong. They are above the blurring effects of the atmosphere (seeing) and can form very precise images. Why is this important? There is a low-level backgournd in the Universe caused by distant galaxies, stars, and gas and dust, which we always pick-up in addition to our targets. If an image is small, we can stop down the opening on our detector and only allow in a small part of the background. If the image is large (because of seeing), we must open up the diaphragm on our detector which allows in more of the background. This makes our detectors much less sensitive for the detection of faint objects. See Section 5.4 for ground-based telecopes which are starting to approach the image quality given by JWST (HST).

Material (Particles) From Stars (14.2) We also study stars using the matter (particles) they produce, e.g., the Solar Neutrino Experiments studies the particle emission from the Sun (see Topic 1). To the right is shown the large Japanese neutrino detector, Super-Kamiokande (Super-K) located in ikenoyama in Gifu-ken. The Nobel Prize in Physics 2002 was awarded to Raymond Davis Jr. and Masatoshi Koshiba for "pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos" and the Nobel Prize in Physics 2015 to Takaaki Kajita and Arthur B. McDonald for "the discovery of neutrino oscillations, which shows that neutrinos have mass."

Gravitational Waves (Chapter 18.4,S3.4) There are also experiments designed to detect the gravitational radiation from compact stars and other Celestial Objects. To the right is shown the Laser Interferometer Gravitational-Wave Observatory (LIGO) located in Hanford, WA which announced the discovery of gravitational waves in 2015 from GW150914, produced by the merger of two orbiting black holes (Abbott et al. 2016). This important work earned the 2017 Nobel Prize in Physics for Rainer Weiss, Barry C. Barish and Kip S. Thorne for "decisive contributions to the LIGO detector and the observation of gravitational waves." Note that there was an earlier indirect detection of gravitational waves for which the Nobel Prize in Physics 1993 was aswarded to Russell A. Hulse and Joseph H. Taylor Jr. for "the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation."

Comment: The opening up of the Electro-magnetic (EM) spectrum and the study of other forms of emissions from Celestial objects have substantially enhanced our understanding of Celestial objects of all kinds and sizes.