What is the prefix with meter to describe the size of the Milky Way?
The Galaxy
About the Image
Image Credit: The Isaac Newton Group of Telescopes, La Palma, and Simon Dye (Cardiff Academy).
Because we dwell within the Milky Way Galaxy, information technology is impossible for us to take a moving picture of its spiral construction from the outside. But we practice know that our Milky Way has a spiral nature from observations made from within our Galaxy (though whether or not it is a barred spiral is nonetheless existence debated). To represent this, the beautiful spiral galaxy Messier 74 was used, as information technology thought to be a similar galaxy to ours.
Below is a pic of the real Milky way taken by the satellite COBE. The disk and heart region of our Galaxy are readily recognizable. This epitome makes the Milky Mode appear much more galaxy-like and less like the smudge of stars we meet stretching across our night sky. It is possible to imagine what our Milky Fashion might look like looking down on it from outside.
Paradigm Credit: The COBE Project, DIRBE, NASA
Distance Information
Although the light twelvemonth is a ordinarily used unit, astronomers prefer a unlike unit called the parsec (pc). A parsec, equal to three.26 low-cal years, is defined as the distance at which 1 Astronomical Unit subtends an angle of 1 2d of arc (1/3600 of a degree) When we use the parsec for actually large distances, nosotros often put a prefix in front of information technology - like kiloparsecs (kpc), which are equal to chiliad parsecs - or Megaparsecs (Mpc), equal to a million parsecs.
The Galaxy is virtually ane,000,000,000,000,000,000 km (about 100,000 light years or about xxx kpc) across. The Sun does not lie near the center of our Galaxy. It lies nigh 8 kpc from the center on what is known every bit the Orion Arm of the Milky Way.
How Practice Nosotros Calculate Distances of This Magnitude
Parallaxes requite usa distances to stars upwards to perhaps a few thousand light years. Beyond that distance, parallaxes are then pocket-size than they cannot be measured with contemporary instruments. Astronomers utilise more indirect methods across a few 1000 light years.
The methods to measure stellar distances greater than a few thousand calorie-free years include:
Proper motions: All stars move across the sky, but but for nearby stars are these motions perceivable, and even then it takes decades or centuries to measure. Statistically, stars move at virtually the same rate; therefore, the stars that appear to have larger motions are nearer. By measuring the motions of a large number of stars of a given class, we can gauge their average distance from their average motion.
Moving clusters: Clusters of stars, such every bit the Pleiades and Hyades star clusters, travel together. Analyzing the apparent movement of the cluster can give u.s.a. the altitude to it.
Interstellar lines: The infinite between stars is non empty, but contains a sparse distribution of gas. Sometimes this leaves absorption lines in the spectrum we observe from stars that prevarication beyond the interstellar gas. (Absorption lines are colors missing in a continuous spectrum because of their absorption by atoms or ions. The spectrum is the array of colors or wavelengths that is obtained when light is dispersed.) The farther a star is, the more absorption will be observed, since the light has passed through more of the interstellar medium.
Inverse-square law: The credible brightness or magnitude of a star depends both on its intrinsic brightness or luminosity (how bright the star really is rather than how brilliant it seems) and its distance from us. The inverse-foursquare law says that the flux from a luminous object decreases as the square of its distance. If we know the luminosity of a star (for instance, we take a measured parallax for one star of the aforementioned type and know that others of the aforementioned type volition take similar luminosities), we can measure its apparent brightness and and so solve for its distance. There are several variations on this, many of which are used to mensurate distances to stars in other galaxies.
Period-luminosity relation: Some stars are regular pulsators, significant their intensity changes periodically. The physics of their pulsations is such that the period of one oscillation is related to the luminosity of the star. If we measure the menses of such a star, we tin can summate its luminosity. From this, and its apparent magnitude, we can summate its distance. The period-luminosity relation was discovered by Henrietta Swan Leavitt in 1908 when she was studying Cepheid Variable stars in the Magellanic Clouds. Cepheids, named afterward Delta Cephei, the first and most luminous of its class to be identified, brand excellent distance indicators, because of their periodicity and boggling brightness. Non only can they be found at the far reaches of our Milky way, they tin can too be resolved in galaxies outside of our own. The well-nigh luminous Cepheids can be used to gauge distances to objects as far equally 12,000,000 light years away.
There are complications in using the flow-luminosity relationship. Commencement, the human relationship itself depends on the chemical limerick of the star. Secondly, the absorption of certain wavelengths of light by the interstellar medium tin affect the apparent effulgence of the star and therefore must be deemed for. Even with these (and other) complications, Cepheid Variables provide an first-class way to measure the relative distances. To convert to absolute distances, nosotros ideally need to measure the distance to a nearby Cepheid with another, more straight, method. At that place is much debate at present in this area, in detail regarding the Hipparcos measurements of distances to nearby Cepheids. (See the Nearest Stars page for more than information on Hipparcos measurements.)
Interestingly, the size of our own Milky way was debated for a long while. It was not until early on in the 20th century that Harlow Shapley used observations of RR Lyrae variable stars to estimate our Galaxy's size. RR Lyrae stars are similar to Cepheid Variables. They accept relatively brusque periods, typically of about a 24-hour interval or less, and all RR Lyrae stars take approximately the same luminosity. Typically, RR Lyrae stars are less luminous than Cepheids, simply they are much more common. Globular clusters of stars - swarms of old stars tightly bound together by gravity and orbiting at the outskirts of galaxies, contain many variable stars, including RR Lyraes.
Shapley was able utilise these to find the distance to the globular clusters that surroundings our Galaxy. Not simply were the globular clusters smashing distances away, merely the Sun did not prevarication at the center of their distribution, which placed the Lord's day far from the center of the Milky way. Shapley'south first guess of the radius of the Milky way was off past a factor of ii, but he made an important first step in understanding the nature of our Milky way.
Several more modern methods take been used to map our Galaxy more than accurately. The neutral hydrogen gas in our Galaxy emits calorie-free at a wavelength of 21 cm; while this lite is invisible to our eyes, it is observable to radio telescopes. Other molecules like carbon monoxide also emit radio waves. This is very helpful for mapping the disk portion of our Milky way.
Why Are These Distances Important To Astronomers?
Distance is a useful tool on the galactic scale. If you can measure out the average speed of stars equally they movement around the Galactic Eye and their distance from the Galactic Middle, you can make a plot chosen a "rotation curve". The rotation curve, which describes the motility of the galaxy can be used to determine the amount of mass within a given radius from the eye. The predicted rotation curves for many galaxies (in particular, screw galaxies similar the Milky Manner) don't match the observed ones, which led to the discovery of dark thing as an caption for this discrepancy. It is thought that these galaxies consist of a large, circular halo of night thing, with the visible thing concentrated in a deejay at its center.
Travel Time
The Voyager spacecraft is traveling away from the Lord's day at a rate of 17.3 km/s. If Voyager were to travel to the center of our Milky way, it would accept more than than 450,000,000 years to travel the eight kpc. If it could travel at the speed of light, an impossibility due to Special Relativity, it would still take over 26,000 years to arrive!
At 17.3 km/s, it would have Voyager over1,700,000,000 years to traverse the entire length of the Milky Way. Even traveling at the speed of low-cal, it would take nearly a hundred thousand years!
Dorsum
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Source: https://imagine.gsfc.nasa.gov/features/cosmic/milkyway_info.html
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