121 lines
2.9 KiB
Plaintext
121 lines
2.9 KiB
Plaintext
-- Title: Show Redshifts of Galaxies
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function get_distance(obj_pos, ref_pos)
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-- Returns distance Earth-object in Mpc.
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distance = ref_pos:distanceto(obj_pos) * km2Mpc
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return distance
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end
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function get_z(distance)
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-- Returns redshift z.
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-- Hubble constant = 73.2 (km/s)/Mpc, WMAP 3years, best
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-- http://map.gsfc.nasa.gov/m_mm/tp_links.html
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local H0 = 73.2
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local c = 299792.458
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z0 = (H0 * distance)/c
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z = z0*(1 - z0/4)/(1 - z0/2)^2
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-- d <= 2 * c/H0
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return z
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end
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function rgb2hex(r,g,b)
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-- Converts color code from RGB to Hex
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local hex = string.format("#%.2x%.2x%.2x", math.floor(r), math.floor(g), math.floor(b))
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return hex
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end
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function get_color(z_rel)
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-- Returns Hex color code from z_rel
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if z_rel > 1 then
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z_rel = 1
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end
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--local green = 255 * (1 - z_rel)/(1 + math.sqrt(z_rel))
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local green = 255 * (1 - z_rel)^2
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local red = 255
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local blue = 255* math.sqrt(green/255)
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hex_color = rgb2hex(red, green, blue)
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return hex_color
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end
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function mark_galaxies(sel_pos)
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zz = z_max(sel_pos)
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for dso in celestia:dsos() do
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if dso:type() == "galaxy" then
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dso_pos = dso:getposition()
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local d = get_distance(dso_pos,sel_pos)
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local z_rel = get_z(d)/zz
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local hex_color = get_color(z_rel)
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dso:mark( hex_color, "disk", 1, 1 )
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end
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end
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end
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function z_max(ref_pos)
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-- determine maximal redshift in catalog wrto ref_pos
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z_old = 0
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for dso in celestia:dsos() do
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if dso:type() == "galaxy" then
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dso_pos = dso:getposition()
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local d = get_distance(dso_pos, ref_pos)
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local z = get_z(d)
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if z > z_old then
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z_max = z
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z_old = z_max
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dsomax = dso
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end
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end
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end
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return z_max
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end
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----------
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-- main --
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----------
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celestia:unmarkall()
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celestia:show("markers")
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km2Mpc = 1/3.08568025e19
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MW = celestia:find("Milky Way")
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MW_pos = MW:getposition()
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--
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-- select and specially mark Milky Way
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--
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celestia:select(MW)
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celestia:mark(MW)
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--
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-- color encode all other galaxies according to their redshift
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-- relative to Milky Way
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--
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mark_galaxies(MW_pos)
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--
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-- move observer to a distance of 1000 Mpc from Milky Way
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--
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observer = celestia:getobserver()
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observer:gotodistance(MW, 1000/km2Mpc,5)
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sel_old = MW
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while true do
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sel = celestia:getselection()
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--
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-- specially mark possible new selection, unmark the old one
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--
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if sel ~= sel_old then
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sel:unmark()
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sel_old:unmark()
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sel:mark("#00FF00","disk",10,1)
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sel_old = sel
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end
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sel_pos = sel:getposition()
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-- obs_pos = observer:getposition()
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if sel:type() == "galaxy" then
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local d = get_distance(MW_pos,sel_pos)
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local z = get_z(d)
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celestia:print(sel:name()..": "..string.format("redshift z = %5.3f, distance = %5.2f Mpc", z,d).."\nmax. redshift: "..dsomax:name(),5,-1,-1,0,6)
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end
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wait(0)
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end
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