Visual astronomy

Completion status: Been started, but most of the work is still to be done.

This image shows the 26-inch Warner & Swasey refracting telescope at the United States Naval Observatory. Credit: Waldon Fawcett.

What is “the “old-fashioned” spirit of real-time visual astronomy”? “I think everyone can conjure up a mental image of astronomers at every level and place in history, gazing through the eyepieces of their telescopes at sights far away - true visual astronomy.”

Educational level: this is a secondary education resource.

"In popular culture optical astronomy encompasses a wide variety of observations via telescopes that are sensitive in the range of visible light. Scientists would call this visible-light astronomy. It includes imaging, where a picture of some sort is made of the object; photometry, where the amount of light coming from an object is measured, spectroscopy, where the distribution of that light with respect to its wavelength is measured, and polarimetry where the polarisation state of that light is measured.", from the former Wikipedia article optical astronomy.

Educational level: this is a tertiary (university) resource.

Educational level: this is a research resource.

Resource type: this resource is an article.

Resource type: this resource contains a lecture or lecture notes.

Subject classification: this is an astronomy resource.

Notation

Notation: let the symbol Def. indicate that a definition is following.

Universals

To help with definitions, their meanings and intents, there is the learning resource theory of definition.

Def. "[t]he natural medium emanating from the sun and other very hot sources (now recognised as electromagnetic radiation with a wavelength of 400-750 nm), within which vision is possible", per Wiktionary light, is called light.

Def. "to shine light on something", Wiktionary illuminate, is called illuminate.

Def. "emitting light", Wiktionary luminous, is called luminous.

Def. "the amount of [visual, or visible light only] electromagnetic energy a body radiates per unit of time", per the Wikipedia article luminosity, is called apparent luminosity.

The overall theory of visual astronomy consists of three fundamental parts:

1. the derivation of visual logical laws,
2. the definitions of visual natural entities, and
3. the definition of the visual sky (and associated realms).

Def. a visual or visible "expanse of space that seems to be [overhead] like a dome" is called a visual sky, or visible sky.

Even in day light, the sky may seem absent of objects if a nearby source tends to overwhelm other luminous objects.

Visible bands

Molecules of "[l]arge polycyclic aromatic hydrocarbons (PAH) ... or their ions are also attractive candidates for the carriers of the diffuse interstellar bands in the visible (DIBs) [because]

1. they have optically active transitions in the visible;
2. they can survive the UV photons in the diffuse interstellar medium; [and]
3. they are the most abundant among the detected molecular species after H₂ and CO."

Visible spectrum

Color	Frequency	Wavelength
violet	668–789 THz	380–450 nm
blue	631–668 THz	450–475 nm
cyan	606–630 THz	476–495 nm
green	526–606 THz	495–570 nm
yellow	508–526 THz	570–590 nm
orange	484–508 THz	590–620 nm
red	400–484 THz	620–750 nm

The spectrum shows the lines in the visible due to emission from elemental hydrogen. Credit:Teravolt.

The spectrum shows the lines in the visible due to emission from elemental helium. Credit:Teravolt.

The spectrum shows the lines in the visible due to emission from elemental carbon. Credit:Teravolt.

The spectrum shows the lines in the visible due to emission from elemental nitrogen. Credit:Kurgus.

The spectrum shows the lines in the visible due to emission from elemental oxygen. Credit:Teravolt.

From the Wikipedia article visible spectrum: "The visible spectrum is the portion of the electromagnetic spectrum that is visible to (can be detected by) the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of frequency, this corresponds to a band in the vicinity of 400–790 THz. A light-adapted eye generally has its maximum sensitivity at around 555 nm (540 THz), in the green region of the optical spectrum (see: luminosity function)."

Below are the visible emission spectra from hydrogen, helium, carbon, nitrogen, and oxygen.

The Balmer series of emission lines from hydrogen occur in the visible spectrum of the Sun at: 397, 410, 434, 486, and 656 nm. Hα is the red line at the far right.

From the Wikipedia article helium: Helium "was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by French astronomer Jules Janssen. Janssen is jointly credited with detecting the element along with Norman Lockyer during the solar eclipse of 1868, and Lockyer was the first to propose that the line was due to a new element, which he named."

Absorption lines

This digitally altered image shows the Fraunhofer lines. Credit: Saperaud.

Per the Wikipedia article astronomical spectroscopy: "Fraunhofer's original (1817) designations of absorption lines in the solar spectrum

Letter	Wavelength (nm)	Chemical origin	Colour range
A	759.37	atmospheric O2	dark red
B	686.72	atmospheric O2	red
C	656.28	hydrogen alpha	red
D1	589.59	neutral sodium	red orange
D2	589.00	neutral sodium	yellow
E	526.96	neutral iron	green
F	486.13	hydrogen beta	cyan
G	431.42	CH molecule	blue
H	396.85	ionised calcium	dark violet
K	393.37	ionised calcium	dark violet"

Fraunhofer line 'A' is an optical astronomy absorption line, not a visual astronomy absorption line.

Violet astronomy

Violet astronomy is conducted over wavelengths between 380 and 450 nm.

Violet photographs of the planet Venus taken in 1927 “recorded two nebulous bright streaks, or bands, running ... approximately at right angles to the terminator” that may be from the upper atmosphere.

"The “Purple Haze” is a diffuse blueish/purple glow within a few arcseconds of the central star in HST images of the Homunculus (Morse et al. 1998; Smith et al. 2000, 2004). This emission is seen in excess of violet starlight scattered by dust, and the strength of the excess increases into the far UV (Smith et al. 2004; hereafter Paper I)."

Blue astronomy

This picture from the Voyager 2 sequence shows two of the four cloud features which have been tracked by the Voyager cameras during the past two months. Credit: NASA.

Blue is the wavelength range 450-475 nm.

From the Wikipedia article on the planet Neptune, "A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue, although Neptune's vivid azure differs from Uranus's milder cyan. Since Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour."

Stars are often referred to by their predominant color. For example, blue stragglers are found among the galactic halo globular clusters. Blue main sequence stars that are metal poor ([Fe/H] ≤ -1.0) are most likely not analogous to blue stragglers.

"[G]round-based UV [and blue astronomy] is a powerful facility for [the] study of [the] chemical evolution of [the] early Galaxy." The UV and B astronomy are over the wavelength range 355.0-500.0 nm.

Also, a topic of blue astronomy is the blueshift change in wavelength.

Cyan astronomy

This is an image of the planet Uranus taken by the spacecraft Voyager 2 in 1986. Credit: NASA/JPL/Voyager mission.

Cyan is 476-495 nm.

From the Wikipedia article Uranus: "In larger amateur telescopes with an objective diameter of between 15 and 23 cm, the planet appears as a pale cyan disk with distinct limb darkening."

Per Uranus: "Methane possesses prominent absorption bands in the visible and near-infrared (IR) making Uranus aquamarine or cyan in color."

Green astronomy

This is a 3D model of the Rosetta Spacecraft. The individual scientific payloads are highlighted in different colours. Credit: IanShazell.

Green objects or emission lines in the green portion of the visible spectrum are the subject of green astronomy.

A picture of the solar corona taken with the LASCO C1 coronagraph. The image is color coded for the doppler shift of the FeXIV 530.8 nm line. Credit: NASA and NRL.

"Carroll and McCormack (1972) in Dublin reported complex spectra in the blue and green wavelength regions of both FeH and FeD".

For elongated dust particles in cometary comas an investigation is performed at 535.0 nm (green) and 627.4 nm (red) peak transmission wavelengths of the Rosetta spacecraft's OSIRIS Wide Angle Camera broadband green and red filters, respectively. "In the green, the polarization of the pure silicate composition qualitatively appears a better fit to the shape of the observed polarization curves". "[B]ut they are characterized by a high albedo." The silicates used to model the cometary coma dust are olivene (Mg-rich is green) and the pyroxene, enstatite.

From the Wikipedia article Stardust (the spacecraft): "In December 2006, seven papers were published in the scientific journal, Science, discussing initial details of the sample analysis. Among the findings are: a wide range of organic compounds, including two that contain biologically usable nitrogen; indigenous aliphatic hydrocarbons with longer chain lengths than those observed in the diffuse interstellar medium; abundant amorphous silicates in addition to crystalline silicates such as olivine and pyroxene, proving consistency with the mixing of solar system and interstellar matter, previously deduced spectroscopically from ground observations; hydrous silicates and carbonate minerals were found to be absent, suggesting a lack of aqueous processing of the cometary dust; limited pure carbon (CHON) was also found in the samples returned; methylamine and ethylamine was found in the aerogel but was not associated with specific particles."

In the image at right the iron (Fe XIV) green line is followed by doppler imaging to show associated relative coronal plasma velocity towards (-7 km/s side) and away from (+7 km/s side) the large angle spectrometric coronagraph LASCO satellite camera.

Yellow astronomy

There are yellow objects and emission lines in the yellow portion of the visible spectrum to introduce yellow astronomy.

During the limb flares of December 18, 1956, a coronal line at 569.4 nm, a yellow line, occurred at 1822 UTC, 1900 UTC, undiminished up to 20,000 km above the solar limb, and at 2226 UTC, is identified as Ca XV. "The coronal temperature was 4000000°." "The December 18, 1956, flare appears to have been a violent condensation of material from a dense coronal cloud above an active region."

Orange astronomy

“Orange light at the wavelength of 590 nm is composed of photons having energy of 2.0 eV.”

The orange system [in orange astronomy] is a number of emission lines very close together forming a band in the orange portion of the visible spectrum. These lines are usually associated with particular molecular species, including ScO, YO, and TiO.

Red astronomy

AZ Cancri. Credit: SDSS Data Release 6.

With respect to the color 'red', there are studies of the 'red shift' and the redness of objects such as the red dwarf AZ Cancri shown in the visual image at right. Red is 620-750 nm.

"Ideally all intrinsic colours should be found from unreddened stars. This is possible for dwarf and giant stars later than about A0 (Johnson, 1964) ... However, it cannot be used for stars of other spectral classes since they are all relatively infrequent in space, and generally reddened."

See also

• Alignment telescope
• Alternative ways to become an observer
• Amateur astronomy
• Astronomy
• Babylonian astronomy
• Backyard Astronomy
• Chinese astronomy
• Dominant group/Astronomy
• Indian astronomy
• Islamic astronomy
• Light and optics
• Mathematical astronomy
• Naked-eye planets
• Observational astronomy
• Pretelescopic astronomy
• Principles of Radiation Astronomy
• Radiation astronomy
• Skygazing
• Solar System
• Solar System and Stellar Astronomy
• Stargazing
• Telescope
• Theoretical astronomy
• Theoretical radiation astronomy
• The visible sky
• Topic:Basic Astronomy
• Topic:Planetary science

Further reading

• Tenorio-Tagle G, Bodenheimer P (1988). "Large-scale expanding superstructures in galaxies". Annual Review of Astronomy and Astrophysics 26: 145–97. 

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