Spectroscopy

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Spectroscopy in Astronomy

Introduction to Spectroscopy

Spectroscopy is a powerful and essential tool in the field of astronomy, enabling scientists to unlock the secrets of the cosmos. By analysing the light emitted or absorbed by celestial objects, astronomers can gain insights into their composition, temperature, motion, and other fundamental properties. This technique has revolutionised our understanding of the universe and continues to be a cornerstone of modern astronomical research.

How Spectroscopy Works

At its core, spectroscopy involves the dispersion of light into its constituent wavelengths, producing a spectrum. This can be achieved using a prism or a diffraction grating. When light from a star or other celestial object passes through these devices, it is separated into its various colours, creating a spectrum that can be recorded and analysed.

Types of Spectra

There are three primary types of spectra that astronomers study:

Continuous Spectrum: Produced by a hot, dense object such as a star or a solid. It displays a continuous range of colors with no distinct lines.
Emission Spectrum: Generated by a hot, low-density gas. It consists of bright lines on a dark background, each corresponding to a specific wavelength of light emitted by atoms in the gas.
Absorption Spectrum: Created when light from a hot, dense object passes through a cooler, low-density gas. It shows dark lines (absorption lines) superimposed on a continuous spectrum, each indicating wavelengths of light absorbed by the gas.

Determining the Composition of Stars

One of the most significant applications of spectroscopy in astronomy is determining the chemical composition of stars. By analyzing the absorption lines in a star's spectrum, astronomers can identify the elements present in the star's atmosphere. Each element has a unique set of spectral lines, acting as a fingerprint that allows for precise identification.

For example, hydrogen, the most abundant element in the universe, produces a series of absorption lines known as the Balmer series. Similarly, helium, sodium, calcium, and other elements each have their characteristic spectral lines that can be detected and analysed.

Additional Insights from Spectroscopy

Temperature

The intensity and distribution of the spectral lines provide information about the temperature of the star. Hotter stars exhibit more intense and broader lines, while cooler stars show narrower and less intense lines.

Radial Velocity

Spectroscopy also enables the measurement of a star's radial velocity—the speed at which it is moving towards or away from us. This is determined by the Doppler shift, where the spectral lines shift towards the blue end of the spectrum (blueshift) if the star is approaching, and towards the red end (redshift) if it is receding.

I used spectroscopy to measure the recession velocity of Quasar 3C273 and my results can be seen here.

Stellar Classification

Stars are classified into different spectral types based on their spectra. The most common classification system is the Morgan-Keenan (MK) system, which categorises stars into types O, B, A, F, G, K, and M, in decreasing order of temperature. Each spectral type is further divided into subclasses (e.g., G2, M5), providing a detailed classification of stellar characteristics.

Spectroscopy in My Work

In my astronomical endeavours, I have utilised spectroscopy to analyze the light from various celestial objects captured in my photos. This technique allows me to determine the chemical composition and physical properties of these objects, providing a deeper understanding of the universe.

For example, by examining the absorption lines in the spectra of stars, I can identify the elements present in their atmospheres. This information helps to reveal the stars' evolutionary stages, temperatures, and motions. Additionally, I have used spectroscopy to study planetary atmospheres, nebulae, and other celestial phenomena, gaining valuable insights into their nature and behaviour.

The technique that I use to acquire the spectra is to insert a diffraction grating to the optical path and then using software to produce the profiles. Real Time Spectroscopy’s RSpec software enables one to analyze starlight. The software can read image files from a CCD camera, live camera input, or a prerecorded video file. It allows the conversion from pixels to angstroms and has a built-in reference library. The software enables one to rapidly go from a static image or video file to a calibrated spectrum graph in real-time.

RSpec's website is a mine of information about producing and analysing spectra and I found the software and the website invaluable.

Conclusion

Spectroscopy is an indispensable tool in the field of astronomy, enabling scientists to uncover the hidden details of celestial objects. By analyzing the light emitted or absorbed by these objects, astronomers can determine their composition, temperature, motion, and more. This technique has vastly expanded our knowledge of the universe and continues to be a fundamental aspect of astronomical research. Through my work, I hope to share the wonders of spectroscopy and inspire others to explore the cosmos.

My Spectroscopy Profiles

Regulus

Regulus is the brightest object in the constellation Leo and one of the brightest stars in the night sky. It has the Bayer designation designated α Leonis, and abbreviated Alpha Leo or α Leo. It lies approximately 79 light years from the Solar System. Regulus is a multiple star system consisting of at least four stars and a substellar object. The largest component, Regulus A, is a B8 type star with the prominent Balmer series of lines clearly visible on the profile.

SAO112406

W Orionis is a carbon star in the constellation Orion, approximately 400 parsecs (1,300 ly) away. It varies regularly in brightness between extremes of magnitude 4.4 and 6.9 roughly every 7 months. When it is near its maximum brightness, it is faintly visible to the naked eye of an observer with good observing conditions. At this stage, it is easy to recognise the typical profile of Carbon Stars.

SAO44317 "La Superba"

La Superba (Y CVn) is a strikingly red giant star in the constellation Canes Venatici. It is faintly visible to the naked eye, and the red colour is very obvious in binoculars. It is a carbon star and semiregular variable. Y CVn is one of the reddest stars known, and it is among the brightest of the giant red carbon stars. The 19th century astronomer Angelo Secchi, impressed with its beauty, gave the star its common name - La Superba.

HIP31579

UU Aurigae is a carbon star in the constellation Auriga. It is approximately 341 parsecs (1,110 light-years) from Earth. It is a variable star that is occasionally bright enough to be seen by the naked eye under excellent observing conditions. Around the star is a shell of dust made up largely of amorphous carbon and silicon carbide (SiC), with the SiC appearing at three times the star's radius and the amorphous carbon at nine times its radius. The profile is typical of these types of stars.

HD50896

Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon. The spectra indicate very high surface enhancement of heavy elements, depletion of hydrogen, and strong stellar winds. The surface temperatures of known Wolf–Rayet stars range from 20,000 K to around 210,000 K, hotter than almost all other kinds of stars. The spectrum shown here, displays the typical strong emissions of carbon, nitrogen etc.

Denebola

Denebola is the second-brightest individual star in the zodiac constellation of Leo. It is the easternmost of the bright stars of Leo. It has the Bayer designation Beta Leonis or β Leonis, which are abbreviated Beta Leo or β Leo. Based upon the star's spectrum, it has a stellar classification of A3 Va, with the luminosity class 'Va' indicating this is a particularly luminous dwarf, a main sequence star that is generating energy through the nuclear fusion of hydrogen at its core. The effective temperature of Denebola's outer envelope is about 8,500 K, which results in the white hue typical of A-type stars.

Aldebaran

Aldebaran is listed as the spectral standard for type K5+ III stars. Aldebaran is a red giant, meaning that it is cooler than the Sun with a surface temperature of 3,900 K, but its radius is about 45 times the Sun's, so it is over 400 times as luminous. As a giant star, it has moved off the main sequence on the Hertzsprung–Russell diagram after depleting its supply of hydrogen in the core. A number of absorption lines are apparent due to Fe (iron), Na (sodium) etc

Carbon Star

A carbon star (C-type star) is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes most of the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere and a strikingly ruby red appearance. The profile in the image is typical of the absorption bands in this type of star.

Herschel H27-4 Planetary Nebula

The really bright green line at 5007 Angstroms cause these PNs to look green to the eye. The line is due to doubly ionized oxygen, so the central star has to be rather hot to ionize the oxygen atoms to O+2. Very "young" PNs that are still getting hotter may not be able to excite this line, and very "old" PNs where the star has cooled off may also not have this line, but a large fraction of them do have this line as the strongest line in the optical spectrum.

Sirius

Sirius is a binary star consisting of a main-sequence star of spectral type A0 or A1, termed Sirius A, and a faint white dwarf companion of spectral type DA2, termed Sirius B. The Balmer series includes four visible spectral lines with wavelengths of 656.3 nm (red), 486.1 nm (blue-green), 434.0 nm (blue), and 410.2 nm (violet). The Balmer series is produced when electrons in a hydrogen atom transition from higher energy levels (n ≥ 3) to the second energy level (n = 2). The image of Sirius shows several Balmer lines together with absorption effects caused by the Earth's atmosphere absorbing oxygen elements. These absorption lines are called Telluric lines. The strong and variable absorption of Earth’s atmosphere interferes with almost all ground-based astronomical observations.

Omicron 1 CMa

The star itself is an orange K-type supergiant of spectral type K2.5 Iab and is an irregular variable star, varying between apparent magnitudes 3.78 and 3.99. A cool star, its surface temperature is around 4,000 K. Around 8 times as massive as the Sun with around 390 times its diameter, it shines with 37,000 times the Sun's luminosity. The Telluric and Balmer series points can be seen here.

HD16523

The more massive a star is, the shorter its lifespan - and Wolf-Rayet stars are among the largest and shortest lived stars known, typically over 20 solar masses with surface temperatures well over 25,000 K. Strong stellar winds eject material from their atmosphere into shells of hot gas which surround the star. Energy radiated by Wolf-Rayets, at levels many orders of magnitude greater than our Sun, peak in the ultraviolet. These ultraviolet rays ionize the gas in these shells to produce the trademark emission lines of the Wolf-Rayet stars.

Spectroscopic Observation of Quasar 3C273

Redshift measurements can help us determine the recession velocity of distant objects like 3C 273, a quasar, by analysing the shift in the wavelengths of light emitted from it.

Here’s how it works:

Understanding Redshift: Redshift occurs when light from an object moving away from the observer stretches, causing its wavelengths to become longer and shift toward the red end of the spectrum. The amount of redshift can be quantified by the redshift parameter, denoted as z.
Measuring Redshift: The redshift (z) is calculated using the formula:

z=(λobserved−λrest)/ λrest

Where:

λobserved is the wavelength of light observed on Earth.
λrest is the wavelength of light emitted by the object at rest (i.e., not influenced by motion).

Recession Velocity: The redshift is directly related to the velocity at which the object is moving away from us. In the case of distant galaxies and quasars, this motion is often due to the expansion of the universe. The recession velocity v is calculated using Hubble’s Law for velocities at cosmological distances, which is approximated by:

v=c⋅z

Where:

v is the recession velocity.
c is the speed of light.
z is the redshift.

Applying to 3C 273: 3C 273 is one of the most famous quasars, and its light shows a significant redshift. By measuring the redshift of the light coming from 3C 273, we can use the above formula to calculate its recession velocity.

In summary, by measuring the redshift of the light from 3C 273, we can use it to calculate the quasar’s recession velocity, revealing how fast it is moving away due to the expansion of the universe.

Several images of the Quasar 3C273 were obtained using a Starlight Xpress SXV-H9 CCD camera with a Paton Hawksley Star Analyser blazed diffraction grating connected to a Meade 250mm Schmidt Cassegrain telescope. The final image comprising 70 exposures, each of 60 seconds, and averaged using Astroart 4.0 is shown below.

The quasar is faint and is only visible at low altitudes from my location so haze and skyglow are factors to be overcome. In addition, although 3C273 is itself faint, when the light is dispersed through the diffraction grating, the spectrum becomes even fainter with the available photons being spread over a larger area.

RSpec was used to convert the spectrum into a calibrated plot as shown below.

The main peaks identified correspond to the Balmer lines, H alpha, H beta and H gamma.

These have rest wavelengths of 6562.82, 4861 and 4340 angstroms respectively.

It can be seen from the above plot that the wavelengths in the observed spectrum are different and have values of 7572, 5635 and 5042 angstroms respectively.

The reason for the difference is due to the “red shift”, where the wavelengths are increased due to the recessional velocity of the observed object.

Distance to 3C273

The distance (z) to 3C273 can be calculated using:

z= Δλ/λo

The average of the above values of z is 0.158

The Simbad database and NED (NASA Extragalactic Database) list the distance as being z = 0.158, so these results are consistent with that value.

The Hubble parameter Ho has a value of approximately 73 km/s/Mpc

Hubble’s Law for calculating the actual distance can be used as:

D = (c * z)/Ho where c is the velocity of light.

This results in a distance to 3C273 of 651 Megaparsec or 2.12 billion light years or (20,000,000,000,000,000,000,000 km)

Recession Velocity

Velocity = c * z, however for extreme cases, to allow for effects predicted by Einstein’s Special Theory of Relativity (such as where z>0.15), a modified formula is used:

V = c * ((z+1)2 – 1)/((z+1)2 +1)

This results in my calculated recessional velocity for 3C273 being 43,695 km/s

The CDS database lists the value as 43,751 km/s