The apparent magnitude of an astronomical object is a number, a measure of its brightness as seen by an observer on Earth. The magnitude scale is logarithmic. A difference of 1 in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The brighter an object appears, the lower its magnitude value, with the brightest astronomical objects having negative apparent magnitudes: for example Sirius at −1.46. The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry. Apparent magnitudes are used to quantify the brightness of sources at ultraviolet and infrared wavelengths. An apparent magnitude is measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V filter band would be denoted either as mV or simply as V, as in "mV = 15" or "V = 15" to describe a 15th-magnitude object; the scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes.
The brightest stars in the night sky were said to be of first magnitude, whereas the faintest were of sixth magnitude, the limit of human visual perception. Each grade of magnitude was considered twice the brightness of the following grade, although that ratio was subjective as no photodetectors existed; this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest and is believed to have originated with Hipparchus. In 1856, Norman Robert Pogson formalized the system by defining a first magnitude star as a star, 100 times as bright as a sixth-magnitude star, thereby establishing the logarithmic scale still in use today; this implies that a star of magnitude m is about 2.512 times as bright as a star of magnitude m + 1. This figure, the fifth root of 100, became known as Pogson's Ratio; the zero point of Pogson's scale was defined by assigning Polaris a magnitude of 2. Astronomers discovered that Polaris is variable, so they switched to Vega as the standard reference star, assigning the brightness of Vega as the definition of zero magnitude at any specified wavelength.
Apart from small corrections, the brightness of Vega still serves as the definition of zero magnitude for visible and near infrared wavelengths, where its spectral energy distribution approximates that of a black body for a temperature of 11000 K. However, with the advent of infrared astronomy it was revealed that Vega's radiation includes an Infrared excess due to a circumstellar disk consisting of dust at warm temperatures. At shorter wavelengths, there is negligible emission from dust at these temperatures. However, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the magnitude scale was extrapolated to all wavelengths on the basis of the black-body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance for the zero magnitude point, as a function of wavelength, can be computed. Small deviations are specified between systems using measurement apparatuses developed independently so that data obtained by different astronomers can be properly compared, but of greater practical importance is the definition of magnitude not at a single wavelength but applying to the response of standard spectral filters used in photometry over various wavelength bands.
With the modern magnitude systems, brightness over a wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30; the brightness of Vega is exceeded by four stars in the night sky at visible wavelengths as well as the bright planets Venus and Jupiter, these must be described by negative magnitudes. For example, the brightest star of the celestial sphere, has an apparent magnitude of −1.4 in the visible. Negative magnitudes for other bright astronomical objects can be found in the table below. Astronomers have developed other photometric zeropoint systems as alternatives to the Vega system; the most used is the AB magnitude system, in which photometric zeropoints are based on a hypothetical reference spectrum having constant flux per unit frequency interval, rather than using a stellar spectrum or blackbody curve as the reference. The AB magnitude zeropoint is defined such that an object's AB and Vega-based magnitudes will be equal in the V filter band.
As the amount of light received by a telescope is reduced by transmission through the Earth's atmosphere, any measurement of apparent magnitude is corrected for what it would have been as seen from above the atmosphere. The dimmer an object appears, the higher the numerical value given to its apparent magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of 100. Therefore, the apparent magnitude m, in the spectral band x, would be given by m x = − 5 log 100 , more expressed in terms of common logarithms as m x
NGC 7025 is a spiral galaxy located about 210 million Light-years away from Earth in the constellation Delphinus. NGC 7025 is classified as a LINER galaxy; the galaxy has an estimated diameter of 161,830 light-years. It was discovered by astronomer Albert Marth on September 17, 1863. List of NGC objects NGC 7013 NGC 7001 NGC 7025 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images
NGC 7013 is a nearby spiral or lenticular galaxy estimated to be around 37 to 41.4 million light-years away from Earth in the constellation of Cygnus. NGC 7013 was discovered by English astronomer William Herschel on July 17, 1784 and was observed by his son, astronomer John Herschel on September 15, 1828. NGC 7013 is tilted 90 ° to the Earth's line of sight. However, NGC 7013 is classified as either as a spiral galaxy with wound arms or as a lenticular galaxy. NGC 7013 is considered part of a class of galactic nuclei, defined by their spectral line emissions, called low-ionization nuclear emission-line region galaxies or LINERs; the galaxy appears to have two rings in its structure. The inner ring appears to disconnect from the central bulge while the stars in the outer ring appear to have little spiral pattern. Optical images of NGC 7013 show that it has a small bulge with a bright inner ring and a faint disk both crossed by dust lanes. A longer exposure of the galaxy made by the Palomar Observatory-National Geographic Sky Survey shows an extended disk around the bulge and the inner ring.
The disk shows little structure except for a faint, thin spiral-like feature running through the galaxy. The neutral atomic hydrogen distribution in NGC 7013 is located in the two rings. In between the two rings there is a low concentration of interstellar medium; the low level of neutral atomic hydrogen in the disk of NGC 7013 and the reddish color of the galaxy suggests that the gas content of the galactic disc has fallen below the threshold at which star formation is to take place. The small bulge-to-disk ratio and the slow rotation velocity show that NGC 7013 is a low-mass, low-density galaxy unlike the more luminous, typical lenticular galaxies; the galaxy may thus be a former late-type spiral galaxy which have exhausted most of its interstellar gas, either by star formation or by internal sweeping. Black Eye Galaxy NGC 4414 List of NGC objects NGC 7020 NGC 7013 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images
North America Nebula
The North America Nebula is an emission nebula in the constellation Cygnus, close to Deneb. The remarkable shape of the nebula resembles that of the continent of North America, complete with a prominent Gulf of Mexico, it is sometimes incorrectly called the "North American Nebula". The North America Nebula is large, covering an area of more than four times the size of the full moon. Binoculars and telescopes with large fields of view will show it as a foggy patch of light under sufficiently dark skies. However, using a UHC filter, which filters out some unwanted wavelengths of light, it can be seen without magnification under dark skies, its prominent shape and its reddish color show up only in photographs of the area. The portion of the nebula resembling Mexico and Central America is known as the Cygnus Wall; this region exhibits the most concentrated star formation. The North America Nebula and the nearby Pelican Nebula are parts of the same interstellar cloud of ionized hydrogen. Between the Earth and the nebula complex lies a band of interstellar dust that absorbs the light of stars and nebulae behind it, thereby determines the shape as we see it.
The distance of the nebula complex is not known, nor is the star responsible for ionizing the hydrogen so that it emits light. If the star inducing the ionization is Deneb, as some sources say, the nebula complex would be about 1,800 light-years' distance, its absolute size would be 100 light-years; the nebula was discovered by William Herschel, from Slough, England, on October 24, 1786 or by his son John Herschel before 1833. Pelican Nebula Media related to North America Nebula at Wikimedia Commons The North America Nebula at the astro-photography site of Mr. T. Yoshida. NASA APOD: The North America and Pelican Nebulae NASA APOD: The North America Nebula NASA APOD: The North America Nebula NGC7000 starpointing.com – Central part of the North America Nebula: The Great Wall North America Nebula on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images
Cosmic distance ladder
The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth; the techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances. Several methods rely on a standard candle, an astronomical object that has a known luminosity; the ladder analogy arises because no single technique can measure distances at all ranges encountered in astronomy. Instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, so on; each rung of the ladder provides information that can be used to determine the distances at the next higher rung. At the base of the ladder are fundamental distance measurements, in which distances are determined directly, with no physical assumptions about the nature of the object in question.
The precise measurement of stellar positions is part of the discipline of astrometry. Direct distance measurements are based upon the astronomical unit, the distance between the Earth and the Sun. Kepler's laws provide precise ratios of the sizes of the orbits of objects orbiting the Sun, but provides no measurement of the overall scale of the orbit system. Radar is used to measure the distance of a second body. From that measurement and the ratio of the two orbit sizes, the size of Earth's orbit is calculated; the Earth's orbit is known with an absolute precision of a few meters and a relative precision of a few 1×10−11. Observations of transits of Venus were crucial in determining the AU. Presently the orbit of Earth is determined with high precision using radar measurements of distances to Venus and other nearby planets and asteroids, by tracking interplanetary spacecraft in their orbits around the Sun through the Solar System; the most important fundamental distance measurements come from trigonometric parallax.
As the Earth orbits the Sun, the position of nearby stars will appear to shift against the more distant background. These shifts are angles in an isosceles triangle, with 2 AU making the base leg of the triangle and the distance to the star being the long equal length legs; the amount of shift is quite small, measuring 1 arcsecond for an object at 1 parsec's distance of the nearest stars, thereafter decreasing in angular amount as the distance increases. Astronomers express distances in units of parsecs; because parallax becomes smaller for a greater stellar distance, useful distances can be measured only for stars which are near enough to have a parallax larger than a few times the precision of the measurement. Parallax measurements have an accuracy measured in milliarcseconds. In the 1990s, for example, the Hipparcos mission obtained parallaxes for over a hundred thousand stars with a precision of about a milliarcsecond, providing useful distances for stars out to a few hundred parsecs; the Hubble telescope WFC3 now has the potential to provide a precision of 20 to 40 microarcseconds, enabling reliable distance measurements up to 5,000 parsecs for small numbers of stars.
In 2018, Data Release 2 from the Gaia space mission provides accurate distances to most stars brighter than 15th magnitude. Stars have a velocity relative to the Sun that causes radial velocity; the former is determined by plotting the changing position of the stars over many years, while the latter comes from measuring the Doppler shift of the star's spectrum caused by motion along the line of sight. For a group of stars with the same spectral class and a similar magnitude range, a mean parallax can be derived from statistical analysis of the proper motions relative to their radial velocities; this statistical parallax method is useful for measuring the distances of bright stars beyond 50 parsecs and giant variable stars, including Cepheids and the RR Lyrae variables. The motion of the Sun through space provides a longer baseline that will increase the accuracy of parallax measurements, known as secular parallax. For stars in the Milky Way disk, this corresponds to a mean baseline of 4 AU per year, while for halo stars the baseline is 40 AU per year.
After several decades, the baseline can be orders of magnitude greater than the Earth–Sun baseline used for traditional parallax. However, secular parallax introduces a higher level of uncertainty because the relative velocity of observed stars is an additional unknown; when applied to samples of multiple stars, the uncertainty can be reduced. Moving cluster parallax is a technique where the motions of individual stars in a nearby star cluster can be used to find the distance to the cluster. Only open clusters are near enough for this technique to be useful. In particular the distance obtained for the Hyades has been an important step in the distance ladder. Other individual objects can have fundamental distance estimates made for them under special circumstances. If the expansion of a gas cloud, like a supernova remnant or planetary nebula, can be observed over time an expansion parallax distance to that cloud can be estimated; those measurements however suf
NGC 7026 is a planetary nebula located 6000 light years away, in the constellation of Cygnus. This image was produced by the Hubble Space Telescope using the Wide Field and Planetary Camera 2 aboard