Knowledge in a Nutshell: Astrophysics

The complete guide to astrophysics, including galaxies, dark matter and relativity

Sten Odenwald

• Publisher: Arcturus
• Year: 2019
• ISBN:
  9781838577575
• Topic: Scienze fisiche
• Subtopic: Astronomia e astrofisica

Whether searching for extra-terrestrial life, managing the effects of space weather or learning about dark matter, the study astrophysics has profound implications for us all. NASA scientist and astronomer Sten Odenwald explains the key concepts of this vast topic, bringing clarity to some of the great mysteries of space.These include:
• The theory of relativity
• Cosmic background radiation
• The evolution of stars
• The formation of the solar system
• The nature of exoplanets
• Space weather systemsFilled with helpful diagrams and simple summaries, Knowledge in a Nutshell: Astrophysics is perfect for the non-expert, taking the complexities of space science and making them tangible. ABOUT THE SERIES
The 'Knowledge in a Nutshell' series by Arcturus Publishing provides engaging introductions to many fields of knowledge, including philosophy, psychology and physics, and the ways in which human kind has sought to make sense of our world.

PART I

The Astrophysicist’s Toolbox

Chapter 1

Observing the Universe

Since the dawn of the printed word, astronomical instruments have dramatically changed in their accuracy, purpose and appearance. From the simple theodolites and cross-staffs used in the 16th century, to the powerful space telescopes of the 21st century, astronomers have used a variety of tools to help them discover what lies beyond the earth.

THE ELECTROMAGNETIC SPECTRUM

One of the most powerful tools for observing the universe is the electromagnetic (EM) spectrum. The electromagnetic spectrum is a collection of photons sorted according to their increasing wavelength, which can be emitted by objects according to a variety of physical processes. By studying this EM radiation you can diagnose the kinds of physical processes taking place. For example, if a source is a powerful emitter of X-rays, you can tell that it contains very hot gases (called plasma) above temperatures of 100,000 k. If the spectrum follows a curved shape called a ‘black body’ you can immediately use this fact to take the temperature of the source. If the shape of the spectrum increases sharply to longer wavelengths, this implies there are electrons within the source travelling at nearly the speed of light within strong magnetic fields. Also, if the light appears as discrete, individual lines of emission, you know that the source is a translucent cloud of gas with emission from individual populations of atoms such as calcium, iron, oxygen and so forth.

The types of telescopes used to gather this EM radiation depend on the wavelength of the photons. At optical wavelengths such as those for which our eyes are sensitive near 500 nanometres, simple lenses and mirrors suffice to focus and reflect the EM energy. At much longer wavelengths measured in millimetres and centimetres, you need the technology of radio receivers in which large metallic parabolic ‘dishes’ are used to focus the radio-wavelength energy.

In addition to detecting faint objects, increasing the aperture of a telescope also greatly improves the resolving ‘power’ of the system. The basic formula for telescopes is

where λ is the wavelength of light in metres and D is the diameter of the mirror (lens) in metres. The human eye has an aperture of about 5 mm (⅙ in) when fully dark-adapted, so at 500 nm its resolution for λ = 500 x 10-9 meters and D=0.005 meters is 30 arcseconds. A 15 cm (6 in) mirror, which is popular for amateur astronomers, can resolve features that are 1 arcsecond in size such as lunar craters 2 km (1¼ miles) in diameter. However, the turbulence and stability of the atmosphere can limit astronomical ‘seeing’ to about 1 arcsecond, smearing out details under twinkling starlight. It wasn’t until the 1990s when computer and servomotor speeds had greatly improved that this ‘adaptive mirror’ technique could be widely employed to eliminate atmospheric twinkling. This technique is so effective that modern ground-based telescopes routinely out-perform the space-based Hubble Space Telescope for certain types of observations.

Telescopes as ‘Light Buckets’

For millennia, we have learned about the universe by using ordinary human eyesight provided by a 5 mm (⅙ in) lens and an organic photodetector called a retina. But by adding a larger lens or mirror an instrument can be created, which greatly increases the number of photons entering the human eye. The single most important purpose of these instruments, called telescopes, is to collect as many photons of light as possible from distant sources, which is a function often referred to as that of a ‘light bucket’. This function is proportional to simply the area of the telescope’s primary objective. Large telescope mirrors (and optical apertures generally) increase the amount of light collected from dim objects allowing them to be studied in detail. The aperture of the human eye is only about 5 mm (⅙ in), and allows us to see stars in the sky as faint as the sixth magnitude (+6m). By increasing the area of the objective lens or mirror, the brightness limit increases by 5 magnitudes for every 100-fold increase in area. Within the neighbourhood of the sun, most stars are between magnitudes of +6m and +15m, while the dimmest stars and galaxies in the visible universe are typically at magnitudes from +20m to +30m. To study them we need the largest apertures we can build to gather their faint light, and this is why astronomers are relentlessly building larger telescopes.

Refracting telescopes use a large objective lens at one end of a cylindrical tube, and a set of smaller lenses at the other end of the cylinder called the eyepiece. The Galileo telescope of 1609 had a magnification of about 21× with an objective lens about 37 mm (1½ in) in diameter, while the largest refractor at the Yerkes Observatory built in 1895 has an objective lens 102 cm (40⅛ in) diameter. Refracting telescopes of any appreciable size are difficult to make because of the number of optical surfaces that need to be precisely polished to focus light. Also, they are supported around their circumference so the massive objective lens of the Yerkes Refractor, which weighs 250 kg (55 lb), sags at its centre, causing optical changes as the telescope is moved. This limitation is the major reason that the construction of large refractors was abandoned in the 20th century.

Reflecting telescopes use a large mirror, or collection of mirror segments, to reflect light to a focus where an eyepiece can be inserted to magnify the image. Since only the front surface of the primary mirror has the required parabolic curve with a reflective surface, the entire rear side of the mirror can be used to support the weight of the mirror without compromising the optics. Isaac Newton’s first reflector in 1668 had a metallic primary mirror, an aperture of 15 cm (6 in) and a magnification of 40x.

The evolution of telescope size showing the equivalent diameter of the primary mirror in metres (vertical axis).

The low cost and exceptional light-gathering ability of reflecting telescopes quickly made them the most desired optical instruments for astronomical research. Initially the primary mirrors were a single piece of glass weighing several thousand kilograms, but a new ‘segmented-mirror’ approach was eventually taken in the late 1970s by combining a dozen or more smaller mirrors into a larger optical framework. This has led to the current Gran Telescopio Canarias whose 36 mirrors provide an aperture with a 10.4 m diameter.

Another technique heavily used in astronomy is called interferometry. By combining the light or radio signals from a source viewed from two telescopes separated by D-metres, you can create a telescope with an effective diameter of D-metres and greatly improve the resolving power of the telescope. Interferometer-based telescopes can now discern details as small as 0.001 arcseconds at radio wavelengths rivalling only the largest optical telescopes. The Very Large Array radio telescope in New Mexico consists of 26 dishes over a baseline some 36 km (22¼ miles) across and can resolve details as small as 0.043 arcseconds.

The VLA interferometer array in New Mexico combines the signals from 26 radio telescopes to create a telescope with very high resolution, which allows photographic-quality radio images to be created of distant nebulae and galaxies.

Meanwhile, some transcontinental Very Long Baseline Interferometry (VLBI) projects involve dozens of individual radio observatories and can achieve a resolution of 10 microarcseconds (0.00001”) at a wavelength of 1.3 mm. Provided the source emits enough energy at these wavelengths, near-photographic images can be created of objects buried deep inside obscuring clouds, or of optically invisible plasma ejected from the cores of distant quasars.

The infrared part of the EM was explored in earnest once sensitive ‘heat’ detectors capable of producing images were developed beginning in the late 1960s. A breakthrough came in 1983 with the advent of the Infrared Astronomical Satellite (IRAS) developed by the European Space Agency with international cooperation. This satellite used sensitive heat sensors, and scanned them across the sky to building up maps of the entire sky at four wavelengths, 12, 25, 60 and 100-microns, revealing a complex universe of multiple sources. The Spitzer Space Telescope launched by NASA in 2003 now gives photographic-quality infrared images of a variety of galaxies and star-forming regions. When combined with radio and optical images of the same source, a truly multi-wavelength perspective can be achieved that reveals the mechanisms behind various phenomena such as star formation and supermassive black hole interactions.

The hidden, dusty arm in NGC 1291 is revealed by the Spitzer Space Telescope, which detects and images infrared radiation from warm dust grains in interstellar clouds.

At shorter wavelengths, we encounter the X-ray universe, which was first discovered in the late 1940’s during rocket launches by an astronomer from the Naval Research Laboratory, Ricardo Giacconi. Interest in this important band continued through the 1960s until the launch of the Einstein X-ray Observatory (HEAO-2) in 1978. This complex instrument was eventually superseded by the Chandra X-ray Observatory launched in 1999. Together, they provided high-resolution images of very energetic celestial sources such as supernova remnants, pulsars, black holes and active galaxies. They also discovered for the first time that infant stars are strong sources of X...