Primary and secondary fluxes

The general picture that emerged from subsequent research is that charged particles (protons with an admixture of heavier atomic nuclei), are accelerated to high energies by processes not yet well determined, but probably in part in supernovae and the shock waves of their aftermath. They then travel through the Galaxy, trapped (stored) therein by the galactic magnetic fields about which they spiral under the influence of the v x B force. Some cosmic rays are so energetic they could not have...

Crosscorrelation or shading method

If there are multiple sources in the field of view, the observed response R'(t) would include the contributions of the several sources. (Here we choose the response function R' that is always positive, Fig. 2d). In the case of a rotating fringe pattern (Fig. 3), the responses of the different sources cancel or reinforce each other depending on the rotation angle of the earth. The modulation would go to zero if two sources (of equal intensities) were perchance modulating exactly out of phase...

Photographs and charts

Palomar, SRC, and ESO sky surveys The true comprehensive charts of the faint stars in the sky at optical wavelengths are actually deep (i.e., sensitive) photographs of the sky. The first and most famous of these is the Palomar Observatory Sky Survey (POSS-I) carried out in the early 1950s. It consists of 936 pairs of large glass plates (350 mm x 350 mm) taken with the large Schmidt telescope at Palomar Mountain in California. The plates cover declinations +90 to -30. At each position, a...

Horizon coordinate system

An observer on the earth's surface at low latitudes notes that the sun rises in the east and sets in the west. During the night, the same observer would note that the stars and planets also rise in the east, move across the sky, and set in the west. These motions are simply an effect of the earth's daily rotation about its axis. For an observer at the earth's north pole, the north celestial pole (NCP) of the celestial sphere is directly overhead (at the zenith). For this same observer, the...

Interplay of observation and theory

The objective of astronomical studies is to learn about the nature of the celestial objects, including their sizes, masses, constituents, and the basic physical processes that take place within or near them. Progress is made through an interplay of observational data and theoretical insight. Observations guide the theorist and theories suggest observations. The pace of this interplay greatly accelerated in the late nineteenth and twentieth centuries due to the rapid increase in technical...

Distances and sizes Distance ladder

The relative sizes of some astrophysical objects and the distances to them are listed in Table 2 and illustrated on logarithmic scales in Fig. 2. The large ranges mentioned above are very evident here. For example, the ratio of the size of the observable 266 9 Properties and distances of celestial objects Table 9.2. Size (radius) and distance examples Solar system Nearest star (Prox. Cen) Crab nebula Center of the Galaxy Galaxy (Milky Way) Andromeda galaxy, M31 Virgo cluster of galaxies 3C273...

Problems

8.2 Unresolved point-like sources Problem 8.21. (a) What is the (partial) luminosity within a narrow 104 Hz bandwidth at frequency v of a radio source radiating isotropically if it is detected at distance 3000 LY with a spectral flux density S 1.0 Jy in this band Repeat for the broad band 50 to 200 MHz if S(v) is constant at 1.0 Jy over this frequency range (b) Another source at the same distance has a spectral flux density S (W m-2 Hz-1) that varies as S kv-2 where k is a constant. The...

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Spherical Pinhole Telescope

Optical light may be collected and focused by means of a transmitting lens that refracts the rays as shown in Figs. 1a and 2a. The disadvantage of a lens in astronomy is that the light must traverse the glass which can lead to imperfect focusing due for example to color dependence of the index of refraction (chromatic aberration). Also, the lenses become very heavy as they become larger. This makes difficult the precision positioning and support of the lens in a movable telescope structure as...

Antenna beams

The concept of an antenna beam is intrinsic to all astronomy. The beam is simply the portion of the sky observed by the detector at a given time (Fig. 5). For example, in a non-focusing detection system, mechanical collimators might restrict the field-of-view to a circular region on the sky of 0.7 radius. The detector would be said to have a 0.7 beam (half width) or 1.4 (full width) that views (0.7)2 1.5 deg2 of the sky. A parabolic radio antenna is a classic example of a focusing system. If...

Thermal and nonthermal radiation

Temperature is one of the most basic quantities in physics and astrophysics. At its simplest, according to the approximation hv kT (2.15), the frequencies of detected photons may be an indicator of the temperature of the originating bodies. For example, photons from the sun at optical frequencies tell us that the surface layers of the sun have temperature of 6000 K (kelvin). The infrared emission from a human being indicates a temperature of about 300 K. A pervasive radio radiation from the sky...

Blackbody radiation

An emitting body can be optically thick. The conditions are such that the photons scatter, or are absorbed and re-emitted, many times prior to being emitted from the surface. In this case one obtains a spectral shape known as the blackbody spectrum. The spectrum depends upon the temperature of the emitting body. Many ordinary objects emit radiation that approximates a blackbody spectrum. Objects at room temperature (T 300 K) emit photons with a spectrum characteristic of T 300 K (infrared...

Spectral flux density

The flux from an unresolved (point-like) object may be considered to be a parallel beam of light or a plane wave originating at infinity. It impinges on the telescope with a given amount of energy deposited per second, per square meter, and per unit frequency interval at frequency v (e.g., in the interval v - 0.5 Hz to v + 0.5 Hz). This is known as the spectral flux density, S(v) Spectral flux density (W m-2 Hz-1) (8.1) but it could be more properly called the spectral energy flux density to...

Radiative transfer equation RTE

The differential equation that governs the absorption and emission in a layer of gas follows from the geometry of Fig. 17. A uniform cloud (source) of temperature Ts, depth A, and optical depth ta lies between the observer and a background source at some other temperature T0. Figure 11.17. Geometry for the radiative transfer equation. The background surface emits with specific intensity I0 and the intervening gas cloud emits thermal radiation with specific intensity Is when it is optically...

Spectral lines

Spectral lines provide powerful diagnostics of the conditions in the emitting region of a celestial source. Normal stars exhibit absorption lines due to decreasing temperature (with altitude) in the photosphere (Fig. 10a) while ejected gas near a star or an active corona can result in emission lines (Fig. 10b). Here we discuss the several types of spectral lines and their measurable characteristics. In the following section, we present the physics of radiation propagation that creates the...

Image formation

Telescopes and antennas are the light collectors of astronomy. They come in varying shapes and sizes that depend in part on the frequency of radiation they are designed to detect. Most systems concentrate the incoming radiation by means of focusing. Optical telescopes gather light with a lens or a reflecting surface (a mirror). Radio telescopes make use of reflecting metal surfaces. X-ray telescopes make use of the reflecting character of a smooth metal surface for x rays impinging on it at a...

Solid angle on the celestial sphere

Solid Angle

The concept of solid angle Q is fundamental to all of astronomy. It is simply an angular area on the sky, or equivalently, on the celestial sphere. This area can be expressed as square degrees or square radians the latter unit is called the The solid angle is expressed in terms of two angular displacements, e.g., d9 and d0 in Fig. 7. The beam of an antenna, e.g., 1 x 1 , or 1x 2, or a fan beam of 1 x 100 can be characterized by its solid angle, approximately 1 deg2, 2 arcsec2, and 100 deg2 for...

Epochs for coordinate systems

The equatorial coordinate system used for celestial measurements depends on the orientation of the earth, and this is a continuously changing function of time (Section 3.2). The time chosen during some period (usually decades) for the specification of celestial coordinates in catalogs and communications between astronomers is called the standard epoch, traditionally expressed in years. The standard epochs in use in the last century, B1900.0 and B1950.0 were based on the Besselian year which...