LOGIC FAMILIES

NON-OPTICAL TELESCOPIC TECHNIQUES USED IN SPACE OBSERVATION (Covering entire Electromagnetic Region)

Astronomers use a number of telescopes sensitive to different parts of the electromagnetic spectrum to study objects in space. For example, different detectors are sensitive to different wavelengths of light. The various observatories used for each band of the EM spectrum are as follows:

Radio observatories

Radio waves can make through the Earth's atmosphere without significant obstacles. Radio telescopes can observe even on cloudy days. So, radio telescopes are not put in space.

A special technique used in radio astronomy is called "interferometry." Radio astronomers can combine data from two telescopes that are very far apart and create images that have the same resolution as if they had a single telescope as big as the distance between the two telescopes. One example is the Very Large Baseline Array (VLBA) that reach from Hawaii to Puerto Rico, nearly a third of the way around the world.

By putting a radio telescope in orbit around Earth, radio astronomers can make images as if they had a radio telescope of the size of the entire planet. The first mission was the Japanese HALCA mission (1997 to 2005). The second mission is the Russian Spektr-R satellite (2011).

Microwave observatories

The Earth's atmosphere blocks much of the light in the microwave band, so astronomers use satellite-based telescopes to observe cosmic microwaves (or CMB).

The first precise measurements of the temperature of the microwave background (i.e. three degrees) across the entire sky was done by the Cosmic Background Explorer (COBE) satellite from 1989 to 1993. Since then, the Wilkinson Microwave Anisotropy Probe (WMAP) operated from 2001 to 2010. More recently, the Planck mission is launched in 2009.

Infrared observatories

While some infrared radiation can make through Earth's atmosphere, the longer wavelengths are blocked. But everything that has heat emits infrared light i.e. the atmosphere, the telescope and even the infrared detectors themselves.

Ground-based infrared telescopes reside at high altitudes in dry climates to avoid water vapor as it absorbs infrared. However, they still account for the atmosphere in their measurements. To get accurate measurement, the infrared emission from the atmosphere is measured at the same time as the measurement of the cosmic object. Then, the emission from the atmosphere is subtracted to get an accurate measurement of the cosmic object. The telescopes are designed to limit the infrared radiation from reaching the detector and the detectors are cooled to limit their infrared emissions.

In 2003, NASA launched the Spitzer Space Telescope.

Another infrared telescope is the Stratospheric Observatory for Infrared Astronomy (SOFIA). SOFIA carries a large telescope inside a 747 aircraft flying at an altitude above most of the Earth's infrared absorbing atmosphere.

                                         Artist's conception of SOFIA flying at sunset

James Webb Space Telescope launched in 2018 is optimized for infrared wavelengths. To keep the mirror and instruments cool (and allow the telescope to detect the faintest of heat signals from distant objects), it has a giant sunshield, which blocks the light and heat from the Earth, Sun and Moon.

Visible spectrum observatories

Visible light can pass through our atmosphere. So we have, ground-based telescope facilities for visible astronomy (optical astronomy). However, as light passes through the atmosphere, it is distorted by the turbulence within the air. Astronomers can improve their image by putting observatories on mountain-tops .

Visible-light observatories in space avoid the turbulence of the Earth's atmosphere. They can also observe a little wider portion of the electromagnetic spectrum, i.e. ultraviolet light which is absorbed by the Earth's atmosphere. The various optical telescopes are as follows:

1. The Hubble Space Telescope in orbit.

2. Kepler observatory in orbit. It is using visible light to survey a portion of      the Milky Way galaxy to discover planetary systems.

3. The Swift satellite which carries an UltraViolet and Optical Telescope (the

    UVOT) to perform observations of gamma-ray bursts.

 Ultraviolet observatories

The Earth's atmosphere absorbs ultraviolet light, so ultraviolet astronomy is done using telescopes in space. Other than carefully-select materials for filters, a ultraviolet telescope is much like a regular visible light telescope. The primary difference being that the ultraviolet telescope must be above Earth's atmosphere to observe cosmic sources.

The GALEX observatory (2003-2013) was the most recent dedicated ultraviolet observatory. Its goal was to observe the history of star formation in our Universe in ultraviolet wavelengths, and it observed over a half-billion galaxies going back to when our Universe was just about 3 billion years old.

The Hubble Space Telescope and the UltraViolet and Optical Telescope on Swift can both perform a great deal of observing at ultraviolet wavelengths, but they only cover a portion of the spectrum that GALEX observes.

X-ray observatories

X-ray wavelengths are blocked by Earth's atmosphere. X-rays are so small and energetic that they don't bounce off mirrors but pass right through. Unless they just barely graze the surface of the mirror.

X-ray telescopes require long focal lengths i.e. the mirrors where light enters the telescope must be separated from the X-ray detectors by several meters. However, launching such a large observatory is costly and is launched in most powerful rockets (the Space Shuttle in the case of the Chandra X-ray Observatory).

In 2012, the Nuclear Spectroscopic Telescope Array (or NuSTAR), designed an observatory with a deployable mast i.e. its mirror module and detector module was designed on a mast, or boom, that was extended once it was in orbit. So, NuSTAR could be launched on a low-cost rocket.

Gamma-ray observatories

Gamma-rays are not only blocked by Earth's atmosphere, but are even harder than X-rays to focus. There have been no focusing gamma-ray telescopes. Instead, astronomers use alternate ways. In this, the properties of the detector can be used or special "masks" can be used that cast gamma-ray shadows on the detector.

The Swift satellite, launched in 2004 has a gamma-ray detector that can observe half the sky at a time, and if it detects a gamma-ray burst, the satellite can quickly point its X-ray and optical telescopes in the direction of the burst.

The Fermi Space Telescope, launched in 2008 is designed to study energetic phenomena from a variety of cosmic sources, including pulsars, black holes, active galaxies, diffuse gamma-ray emission and gamma-ray bursts.

Astronomers can use ground-based astronomy to detect the gamma-rays. The telescopes don't detect the gamma-rays directly. Instead, they use the atmosphere itself as a detector. The HESS array has been in operation for over 10 years. The array began with four telescopes arranged in a square, and recently added the HESS II telescope.

Newton’s Law of Gravitation

Newton's law of universal gravitation states that every particle attracts every other particle in the universe with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.

Presently,  the law states that every point mass attracts every other point mass by a force acting along the line intersecting the two points. The force is proportional to the product of the two masses, and inversely proportional to the square of the distance between them.

The equation for universal gravitation thus takes the form:

{\displaystyle F=G{\frac {m_{1}m_{2}}{r^{2}}}\ } 

                         

where F is the gravitational force acting between two objects, m₁ and m₂ are the masses of the objects, r is the distance between the centres of their masses, and G is the gravitational constant (6.674×10⁻¹¹ N · (m/kg)²).

Newton's law of gravitation resembles Coulomb's law of electrical forces, which is used to calculate the magnitude of the electrical force arising between two charged bodies. Newton's law has since been superseded by Albert Einstein's theory of general relativity, but it continues to be used as an excellent approximation of the effects of gravity in most applications. Relativity is required only when there is a need for extreme precision, or when dealing with very strong gravitational fields, such as those found near extremely massive and dense objects, or at very close distances (such as Mercury's orbit around the Sun).

Determining Distances in Space

There are several techniques for measuring distances in space:

1. Geometric Methods  –For close stars

2. Spectroscopic Measurements –Close to medium distance stars (most of the stars in our galaxy)

3. Use of “Standard Candles” –For very bright objects in distant galaxies

4. Using Redshifts. 

Geometric Methods

1)Parallax: - It provides the distance in parsec pc (parallax arcsecond).

2)Angular Size

 If we know the size of an object and what angle it takes up in the sky, then we can calculate how far it is.

 
When D>>d , 𝜹rad ≈ (d/D)
                    𝜹degree ≈ (180°/π)(d/D)
                    𝜹degree ≈ 57.2958°(d/D)
                    𝜹'arc min ≈ 3438' (d/D)
                    𝜹''arc sec ≈ 206265 (d/D)

                     In case of moon, 
                                                          𝜹 = 0.5° or 31.08 arc min or 1865.18 arc sec
                            Radius of moon (d/2) = 1732 km
                                                           d  =  3476 km
          Therefore, Distance of moon (D) = (3476 ✕ 206265) /1865.18
                                                               =384400 km

     Spectroscopic Measurements

If we know an object’s apparent magnitude, and its absolute magnitude, we can calculate how far away it is.

Stellar “Candles”

For distances which are too large, astronomers use 'standard candles'. Light sources which are further away appear fainter because the light is spread out over a greater area.  If we know how luminous a source really is, then we can estimate its distance from how bright it appears from Earth. The light which reaches Earth has spread out over a sphere.

Radius of sphere = distance to earth, r

Surface area of sphere = 4πr ²

On Earth, the received power per unit area is then

received power per unit area = source luminosity/area of sphere 

                                           P = L /4πr ²

Using Redshifts

An object that is redshifted will have its peak brightness appear  towards the red end of the spectrum.

This is calculated with an equation, 

z = (λ_observed - λ_rest)/λ_rest  where

z is redshift parameter.

λ_observed is the observed wavelength of a spectral line.

λ_rest is the wavelength that line would have if its source was not in motion.

z tells the number of years the light from the object has traveled to reach us, however this is not the distance as the universe has been expanding as the light traveled and the object is now much farther away.

z	Time the light has been travelling	Distance to the object now
0.0000715	1 million years	1 million light years
0.10	1.286 billion years	1.349 billion light years
0.25	2.916 billion years	3.260 billion light years
.5	5.019 billion years	5.936 billion light years
1	7.731 billion years	10.147 billion light years

luminosity of star

Luminosity is the total amount of energy emitted per unit of time by a star. In SI units luminosity is measured in joules per second or watts.

A star’s luminosity depends on two things:

1. Radius measure
2. Surface temperature

Radius measure

If a star has the same surface temperature as the sun, but its radius is 4 solar (4 times the sun’s radius). Then its luminosity with respect to Sun is,

L is propotional to R²

L is propotional to 4² = 4 x 4 = 16 times the sun’s luminosity

where L = luminosity and R = radius

Surface temperature

Also, if a star has the same radius as the sun but its surface temperature is twice as that of Sun (5800 x 2 = 11600 Kelvin).

Then star’s luminosity, relative to the sun is,

L is propotional to T⁴
L is propotional to 2⁴ = 2 x 2 x 2 x 2 = 16 times the sun’s luminosity.

where L = luminosity and T = surface temperature

Luminosity of Star is propotional to R² x T⁴

The luminosity of any star is the product of the radius squared times the surface temperature raised to the fourth power. Given a star whose radius is 3 solar and a surface temperature that’s 2 solar, star’s luminosity is,

L is propotional to R² x T⁴
L is propotional to (3 x 3) x (2 x 2 x 2 x 2)

L is propotional to 9 x 16 = 144 times the sun’s luminosity

where L = luminosity, R = radius and T = surface temperature.