Spectroscopy
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:
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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.
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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.
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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
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:
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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.
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Measuring Redshift: The redshift (z) is calculated using the formula:
z=(λobserved−λrest)/ λrest
Where:
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λobserved is the wavelength of light observed on Earth.
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λrest is the wavelength of light emitted by the object at rest (i.e., not influenced by motion).
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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:
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v is the recession velocity.
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c is the speed of light.
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z is the redshift.
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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
